How to reconcile Mahayana ideas of "nama-rupa" with the Pali definition? I read the following ideas on the internet by Mahayana faithful:


  Namarupa means "name-form" - meaning "a concept of form", "an idea of
  form" - referring to our subjective representations of external and
  internal phenomena, as well as the most important Name-Form, our idea
  of self. So in my understanding, every time we delineate [external!] objects, our ideas of objects keep getting more concrete. And then these ideas feed back into the delineation process, making delineation more precise but also more rigid and fixed. Thus these two - "the process of delineation" and "the collection of ideas" - support each other in their growth and development.


And, in affirming the above idea and negating an alternate explanation, another Mahayana said:


  But your understanding of consciousness and name-form is not Buddhist.
  It's not even logically correct.


Now, in the Pali suttas of SN 12.2 and MN 9, "nama-rupa" is described as follows:


  And what are nama and rupa?
  
  Katamañca, bhikkhave, nāmarūpaṃ?
  
  Feeling, perception, intention, contact and attention.
  
  Vedanā, saññā, cetanā, phasso, manasikāro—
  
  This is called nama.
  
  idaṃ vuccati nāmaṃ.
  
  The four primary elements and form derived from the four primary
  elements.
  
  Cattāro ca mahābhūtā, catunnañca mahābhūtānaṃ upādāyarūpaṃ.
  
  This is called rupa.
  
  Idaṃ vuccati rūpaṃ.


In his translation of the Majjhima Nikaya in 1995, Bhikkhu Bodhi translated 'nama-rupa' as 'mentality-materiality' as follows:


  When, friends, a noble disciple understands mentality-materiality, the
  origin of mentality-materiality, the cessation of
  mentality-materiality, and the way leading to the cessation of
  mentality-materiality, in that way he is one of right view…and has
  arrived at this true Dhamma.
  
  Feeling, perception, volition, contact, and attention—these are called
  mentality. The four great elements and the material form derived from
  the four great elements—these are called materiality.
  
  MN 9


Also, in the Pali, the word "nimitta" is often found, to mean "theme" or "sign". For example, ideas such as "beautiful" are said to be a "nimitta". The Pali (MN 43) says such nimitta are produced by greed, hatred &amp; delusion. Therefore, the impression is these nimitta are much more than mere "perception" ("sanna"). At least in the Pali, "nimitta" appear to be "mental formations" ("sankhara"). 

My questions are:


How does the Pali definition above about "feeling, perception, intention, contact and attention" describe the Mahayana idea of "a concept of form" and "an idea of form"? Particularly how do the Pali terms "contact", "intention" and "attention" operate as part of this Mahayana "a concept of form" and "an idea of form"?
How is the Mahayana "a concept of form" and "an idea of form" different to the Pali "nimitta"?
Unlike Theravada tradition, which passes down the words, Mahayana tradition passes down the meaning - using whatever words and images that are most fitting for the occasion and the recipient.

So in Mahayana tradition there is a large number of texts that explain the major points of Buddhism in a variety of ways. Some are written as poems, some are as formal treatises, some are as stories and anecdotes, and some are expansive comments elaborating on topics arranged in methodical sequence. Also, in Tibetan Mahayana tradition, it is customary for a tutor to go over an ancient (Mahayana) text, explaining the overall meaning and commenting on the points made, so that the next generation does not get stuck with a text they no more understand.

Many of these texts and lectures cover development and arising of the samsaric mind. Because Mahayana is more focused on the overall meaning than on the exact form, many of these teachings do not literally go point by point over the Twelve Nidanas, but they do describe the overall process, some in more broad terms and some in more details. One such example is Lamp of Mahamudra by Tsele Natsok Rangdrol which I quote in this answer. Another example is Thrangu Rinpoche's lecture on Madhyantavibhanga. Yet another example is Chogyam Trungpa's lecture that I quote in this answer.

Not a single of these explanations is 100% perfectly clear, but combined together they absolutely paint the coherent picture.

I have read and was exposed to enough of these learning materials on the topic of development and arising of the samsaric mind that I now have a very strong confidence that I do understand the overall meaning.

The gist of Dependent Origination (a better translation would be "Gradual Auto-Arising"), as explained in these texts and by these teachers, is progressive/incremental/gradual development of differentiation, or development of the mind of delineation and designation of entities. This process starts from complete non-differentiation, goes through a phase of vagueness or semi-ambiguity, then matures to the point when individual objects are designated, and all this time in parallel with development of object-thinking there is a growing separation of this vs that, or self vs world. As this idea of self matures it starts getting involved in progressively more active interactions with the world of objects, until through the act of formulating an intention, carrying it out, appropriating the result, and connecting these three steps together, it gets enough cohesion as to become self-aware. Most explanations understandably paint this process in dark colors by showing how the increased separation leads to fear, aggression, indulging in pleasures, and eventual imprisonment in a web of results, relationships, obligations and other karmic constraints.

It is rather clear to me from these explanations that Namarupa-Nidana corresponds to an early phase of this process, when the mind has just accumulated enough impressions and tendencies as to start differentiating something, but before this something gets integrated across all sensory modalities as to become an "object" we can "come in contact" with. So Namarupa is a phase when the mind builds its notions of objects as cohesive entities. Chogyam Trungpa, for example, describes this phase so:


  Things slowly escalate that way. ... When an object has a conceptualized name, it becomes significant. ... Names and forms serve as ... philosophical reinforcement. For instance ... you think that the title should fit the person.


From many such explanations, plus the very etymology of nama-rupa itself, plus my own years of meditation experience, it is rather clear to me that namarupa stands for some sort of information about an object's identity.

Or, perhaps, Namarupa stands for the entire collection of such information, comprising a single individual, what today we would call the "psyche". That is a possibility. It could even stand for both. (Remember the situation with the word Dharma which means phenomena, things, thoughts, a way of life, or the Buddha's Teaching - depending on the context.) In any case, Namarupa is definitely of the nature of information about entities and identities, names and forms.

Now, to answer your concrete questions:


  
  How does the Pali definition above about "feeling, perception, intention, contact and attention" describe the Mahayana idea of "a concept of form" and "an idea of form"? Particularly how do the Pali terms "contact", "intention" and "attention" operate as part of this Mahayana "a concept of form" and "an idea of form"?
  


The point here is that the Skandhas (MN 9 is about the only sutta that erroneously brings in the "contact") are a collection of information, they are nothing else but Namarupa. As was explained many times, e.g. in Questions of King Milinda, the distinct Skandhas are a product of logical analysis, not actually separate things. In actuality the "stuff" that is skandhas is not divided into the four or five neat "piles of brushwood", it is all mixed together into what's traditionally described either as burning fire or the ocean where all rivers' waters mix. So the skandhas are "made" from the same "material" as the previous nidana, just a bit more specialized by now. What used to be formations(tendencies,latencies) solidifies into ability to delineate stuff, which eventually matures into a collection of information about the world of objects and the corresponding "psyche" or the inner world. This is Namarupa.


  
  How is the Mahayana "a concept of form" and "an idea of form" different to the Pali "nimitta"?
  


Nimitta is what's known in cognitive sciences as identifying feature.

(Nimitta is not a "theme", that's a mistaken translation. The example of "a nimitta of beauty" speaks about a sign of beauty, such as for example long hair or big eyes or nice proportions or whatever.)

Instead, Nimitta is some basic clue that the mind uses to recognize stuff in the stream of raw sensory input. Namarupa is a more general concept, it is a notion of object, which may have one or more identifying features that (either individually or together) help us recognize the object.

Remember, according to Mahayana, objects do not exist, they are empty. Objects are constructs of mind; they are projections. Mahayana speaks about it all the time.

So, nimitta is just one part of this story, it is the sign by which an object is identified. When the sign is latched on, there come the chain of associative perception, the associated feelings, the associated impulses to do something in regards with the object etc. and all this information-about-the-object-and-the-mental-activity-it-enables is called Namarupa. I hope it's clear now.