Mik,
Your questions may depend on if you are following Sefardi minhagim or ashkenaz ones. In response to your first question, some say that you should not answer amen “beemtza haperek” – in middle of a paragraph. Thus, if you are in the middle of ashrei, it is not so simple that you can just answer. I would suggest, if you see the Chazzan going to the bima, finish whatever paragraph you are up to and wait to answer amen. Note that if you finish Yehi chevod, you will have to repeat the last passuk – vehu rachum again because you are supposed to connect the end of yehi chevod to ashrei.
For your second question, the above posts were correct but I am not sure how clear they were. If you are not finished the first yehiyu leratzon (after sim shalom), you must continue without answering amen to anything and once kedusha comes, you must pause and listen and follow the kedusha without saying it. Then continue after kedusha is finished. If you did finish the yehiyu leratzon, you can answer amen and baruch hu ubaruch shemo as well as kedusha even in middle of elokai netzor.
As to your third and fourth questions, the other two posts sound correct according to all shitos (as far as I know).
I am not a posek so I am not saying everything I said was correct but at least this can give you an idea of the applicable halachos.
Hatzlacha with everything and Hashem should give you tremendous bracha for becoming a baal tshuva.