Bhava Tanha & Vibhava Tanha The above mentioned technical terms are respectively translated as Craving for Becoming (something) &amp; Craving for Non-Becoming/Getting Rid Off.

Above said, how do we know that we're in Craving for Becoming (or not) whenever we're cultivating good? One of the limbs of the N8P, that is, 'bhavana', is about active cultivation. Where is the difference between the two?

Similarly, if we do virtuous acts and reflect on those acts regularly, the perception of a 'virtuous person' increases as well, doesn't it? This too, then, is Bhava Tanha?

Now with Vibhava Tanha:  Suppose I am overweight and must lose weight immeditately; is the 'getting rid' of my body fat then considered vibhava tanha and at the same time 'bhava tanha' (becoming a skinny person)?

Finally, when do we exactly know whether something is bhava tanha or kusala chanda? Is it primarily our motivation, i.e., in this case, for whom we're losing weight/what motivation causes us to lose weight, or is it something that we can't avoid at all, since 'identity birth(s)' ('I am this', 'I am not this') exists in everyone except in a fully enlightened being.
Similarly, if we do virtuous acts and reflect on those acts regularly,
  the perception of a 'virtuous person' increases as well, doesn't it?
  This too, then, is Bhava Tanha?


Dana is the perfect case to illustrate that merit depends on ''good deeds'' and the intention behind them, as usually claimed by the buddha. Never forget that deeds (thought, speeches, actions) are always tied to intentions (or rather to cetayanas). This is the novelty claimed by the buddha, instead of normal people who think that virtue lies either all on the side of deeds, or all on the side of intentions, or some even say that virtue does not exist.

that's the relevant texts about giving:

https://www.accesstoinsight.org/tipitaka/an/an07/an07.049.than.html

https://www.accesstoinsight.org/tipitaka/an/an06/an06.037.than.html

https://www.accesstoinsight.org/lib/authors/various/wheel367.html

The dana of something to a ''renunciant'' (bikkhu or lay person) is always good, but the fruit of this deed depends on how the giver view the deed.  there are 8 good intentions listed in the famous sutta about dana


there is the famous case, where the giver focuses on meritorious rebirth
there is a famous case, where the giver gives stuff to an ascetic by tradition
there is the famous case where the giver thinks that ''giving is good because it means  you care about other people'' and rich people must give riches to poor people


Then there is the most important case where the giver focus on his citta, because, by right view and right intention, dana is part of right action. People who cling to the famous views above do not have access to this higher view.

So   there is Bhava Tanha when you do act meritoriously, but do not have right intention (= nekkhamma and metta), ie when you do not understand why dana is good (and not dana is bad). This is normal for puthujjanas since at best they memorize right view, instead of knowing right view.
Puthujjanas who cling to their view that nibanna means loving everybody and being selfless are stuck at the level of ''giving is good because it means  you care about other people'', and they think that caring about their citta is bad. In short, they worry to much on what the recipient feels and enjoys. They do not understand that the cultivation of right intention is about their citta and that metta alone is not right intention... 
Right intention is metta+Nekkhamma. It is metta which gets you in meritorious states, but It is Nekkhamma which makes you avoid bhava Tanha.

The way to have sila, respect the precepts and all is precisely mindfulness. As long as you are mindful, you are assured to have right intention (= nekkhamma and metta). This is why mindfulness matters :


  Being heedful, he achieves consummation in virtue.


https://www.accesstoinsight.org/tipitaka/an/an10/an10.015.than.html

or even with Nanda here 

http://obo.genaud.net/dhamma-vinaya/ati/an/08_eights/an08.009.than.ati.htm 

It is the same thing with kusala chanda. When there is a buddha and people are inspired by him,  there is kusala chanda. When there is no buddha, it gets harder to have kusala chanda, because puthujjanas love to idolize people and do like their idols, so when there is no buddha, they only idolize only other puthujjanas and they make their life worse.

So 3 taṇhā you talk about, are about the object, kama, bhava, vibhava. But the object of right intention is indeed the samadhi of the citta, nibanna, dispassion, seeing things as dukkha and so on.