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.
I don’t think this is true. You can say Amen at any point in Pesukei D’zimra because it’s part of Shevach to HKBH. There are some Shittos that you can even answer Amen in the middle of the Brachos of Baruch Sheamar and Ashrei. Can you please bring a source for this opinion?
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.
This paragraph is self-contradictory and also not so clear. It is a Machlokes Haposkim whether or not you can say Amen in the middle of Elokai Netzor. The Minhag in many places is like the Kitzur Shulchan Aruch that you can. Same with Kedusha. Once you say the second Yihyu Leratzon then you are done with Elokai Netzor and can answer anything no matter what.
Just a note about “saying Kedusha”. When we say you can say Kedusha that means only the Pesukim of Kadosh Kadosh Kadosh and Baruch Kevod. It doesn’t mean the paragraphs (or phrases) in between or Yimloch Hashem (and certainly not Shema Yisrael on Shabbos).