Form in the formless realms Dependant Origination declares that from Consciousness (Viññana) arises Nāma-Rūpa, not just Nāma. Therefore it is implied that even in formless realms the entities that exist there also have a form of some kind.
Uddakka Ramaputta and Alara Kalama were most probably born in formless Brahma realms after their death.
But Buddha didn't visit Alara-Kalama and Uddakka-Ramaputta which means that they couldn't understand the Dhamma which must have been a problem with the senses or something similar (My first question) because they already had superior mental ability to understand the Dhamma (MN 21),

Alara Kalama is wise, competent, intelligent. He has long had little
dust in his eyes. What if I were to teach him the Dhamma first? He
will quickly understand this Dhamma.

Dust here implies the 5 hindrances.

This Uddaka Ramaputta is wise, competent, intelligent. He has long had
little dust in his eyes. What if I were to teach him the Dhamma first?
He will quickly understand this Dhamma.

I would like to get an answer to either or both (if possible) of the below questions,
Question 1: Why couldn't Buddha teach the Dhamma to Uddakka Ramaputta and Alara Kalama.
Question 2: What is the difference in form between the Rūpa Lōka and Arūpa Loka.
OP: Question 1: Why couldn't Buddha teach the Dhamma to Uddakka Ramaputta and
Alara Kalama.

The reason for this is that they had already passed away at the time of the Buddha's enlightenment.
From MN 26:

&quot;Then the thought occurred to me, 'To whom should I teach the Dhamma
first? Who will quickly understand this Dhamma?' Then the thought
occurred to me, 'This Alara Kalama is wise, competent, intelligent. He
has long had little dust in his eyes. What if I were to teach him the
Dhamma first? He will quickly understand this Dhamma.' Then devas came
to me and said, 'Lord, Alara Kalama died seven days ago.' And
knowledge &amp; vision arose within me: 'Alara Kalama died seven days
ago.' The thought occurred to me, 'A great loss has Alara Kalama
suffered. If he had heard this Dhamma, he would have quickly
understood it.'
&quot;Then the thought occurred to me, 'To whom should I teach the Dhamma
first? Who will quickly understand this Dhamma?' Then the thought
occurred to me, 'This Uddaka Ramaputta is wise, competent,
intelligent. He has long had little dust in his eyes. What if I were
to teach him the Dhamma first? He will quickly understand this
Dhamma.' Then devas came to me and said, 'Lord, Uddaka Ramaputta died
last night.' And knowledge &amp; vision arose within me: 'Uddaka
Ramaputta died last night.' The thought occurred to me, 'A great
loss has Uddaka Ramaputta suffered. If he had heard this Dhamma, he
would have quickly understood it.

The sutta proceeds with the Buddha selecting the five monks at the Deer Park at Isipatana, to be his first disciples. They are the ones who attended to him when he practiced extreme asceticism.


OP: Question 2: What is the difference in form between
the Rūpa Lōka and Arūpa Loka.

If you mean the traditional Buddhist cosmology of the 31 planes of existence, please see the AccessToInsight page on The Thirty-One Planes of Existence:

The Immaterial World (arupa-loka). Consists of four realms that are accessible to those who pass away while meditating in the formless
jhanas.
The Fine-Material World (rupa-loka). Consists of sixteen realms whose inhabitants (the devas) experience extremely refined degrees
of mental pleasure. These realms are accessible to those who have
attained at least some level of jhana and who have thereby managed
to (temporarily) suppress hatred and ill-will. They are said to
possess extremely refined bodies of pure light. The highest of these
realms, the Pure Abodes, are accessible only to those who have
attained to &quot;non-returning,&quot; the third stage of Awakening. The
Fine-Material World and the Immaterial World together constitute the
&quot;heavens&quot; (sagga).
The Sensuous World (kama-loka). Consists of eleven realms in which experience — both pleasurable and not — is dominated by the five
senses. Seven of these realms are favorable destinations, and include
our own human realm as well as several realms occupied by devas. The
lowest realms are the four &quot;bad&quot; destinations, which include the
animal and hell realms.

The details of all 31 planes are on that page.


OP: Dependant Origination declares that from Consciousness (Viññana) arises Nāma-Rūpa, not just Nāma. Therefore it is
implied that even in formless realms the entities that exist there
also have a form of some kind.

If you look at that page, it says that unconscious beings (asaññasatta) who are on plane #22 have body but no mind. On the other hand, the top four types of beings in the arupa-loka have mind, but no body.
Doesn't that conflict with the definition of name-and-form in Dependent Origination?
My take on it is that the Dependent Origination taught in the Pali suttas applies to humans, and any beings with both body and mind. The Buddha, being a pragmatic teacher, targeted this teaching only at those who are able to listen to it and understand it. This does not include the unconscious beings (asaññasatta), those beings in the arupa-loka, and also the beings who are in the state of deprivation (apaya).
There may be more to Dependent Origination as in how it may apply to those beings with mind and no body, or with body and no mind, but this is one of those things which the Buddha did not teach us because it is not useful to us.
As we see in SN 56.31:

Once the Blessed One was staying at Kosambi in the simsapa forest.
Then, picking up a few simsapa leaves with his hand, he asked the
monks, &quot;What do you think, monks: Which are more numerous, the few
simsapa leaves in my hand or those overhead in the simsapa forest?&quot;
&quot;The leaves in the hand of the Blessed One are few in number, lord.
Those overhead in the simsapa forest are more numerous.&quot;
&quot;In the same way, monks, those things that I have known with direct
knowledge but have not taught are far more numerous [than what I have
taught]. And why haven't I taught them? Because they are not connected
with the goal, do not relate to the rudiments of the holy life, and do
not lead to disenchantment, to dispassion, to cessation, to calm, to
direct knowledge, to self-awakening, to Unbinding. That is why I have
not taught them.


There is an alternative view to this.
The alternative view is that the 31 planes of existence are just different states of mind, that is to say not a literal physical rebirth into those planes, but rather the momentary rebirth of one's personhood into those planes i.e. entering different states of mind. This is usually supported by the association of the 4 rupa jhanas and 4 arupa jhanas, as well as brahmaviharas with different planes.
This is like a more subtle perspective into the teachings, compared to the one previously mentioned, which is gross.