While understanding Rama's actions we should keep in mind that Rama is embodiment of Dharma itself.
"Ramo Vigrahavan Dharma"
You can understand the same while solving public complaints in his time. For example, A dog's complain against a sadhu, and a quarrel between owl and eagle in Ramayana Uttarakhanda. He is not just expert in dharma, he is the dharma. Situations like abandonment of mother Sita, cannot be understood with our simple logical brains where we don't even have least idea of what dharma means and don't ask me about following it.
Anyway as you asked here for an answer and I will provide one which I learnt from my guru by the grace of his lotus feet.
Dharma is not a stubborn thing like something carved on a rock. It depends on both time and space. This situation of Rama's dharma is complicated here. Where his dharma as king of the whole earth and as a husband got conflict. In situations like this we should keep one of them away "for sometime" then take it back when the time is right. And Rama wanted to take mother Sita back in front of everyone with testimony given by Maharshi like Valmiki in Ashvamedha yaga."
If you read Ramayana without a negative attitude you will understand that either Rama or Lakshmana did'nt have even slightest doubt on her.
Durvasa Maharshi told king Dasharadha that according to jataka, "Rama has to live away from all his dear ones soon or later. Oh king, you will also loose Rama in no time." Soon after this meeting Rama would leave for forest.
Regarding the other two incidents you mentioned:
Ahalya: usually we think of these kind of incidents as punishment. If you look at them, they are actually methods of getting rid of those sins and to get a better state in spirituality or may be moksha itself. What she got at the end of her "so called punishment" she got darshanam of sat-chit-ananda in human form. So how can it be a punishment?
Tara: First of all Sugriva and family are not humans at all. Then how come you ask them to behave according to human code of law? Again if you read Kishkindakhanda "without a negative attitude" you will understand that it's a rule in kishkinda that in case of death of a bother his brother has to take care of the dead ones family as his own. It includes even considering the other's wife as his own.