Sorry if this question has already been answered but I haven´t found it.

Here is a situation. I have a base on the planet and I have air defense module. Opponent conquers my base and his units are on the same area as my base. Than another opponent who doesn´t have an offensive module wishes to attack these units in my base from another planet. Is this possible? From my point of view unfortunately not because I still have the base so limited orbital defense is still functional and I unintentionaly provide cover for opponent who conquered my base. Another question is will he (oppponent who conquered my base) get +1 bonus against flying units and detector in case of being attacked? I don´t think so because I am the owner of the base but I want to make sure about these two questions. Could you please tell me whether my interpretation is correct?

You are correct on both counts.

Your air defence module (ADM) prevents others from landing in the area with a base. Unless they have and choose to use the offensive module to bust in there.

And with regards to the +1 vs Air units from the ADM, you need to own the base in the area to get the advantage. So you are correct, neither of the other players can gain this advantage from your base being present there.