mik5…
1) if you are on ashrai, yes you do answer amain.
2) if you are saying amidah you do not answer amen or say kedusha with the minyan, you just pause and listen to the chazzan, UNLESS you have already finished the ‘Sim Shalom’ paragraph at the end of the amidah. THEN you CAN say kedusha with the minyan, and continue with ‘E-lokai netsor…’ after kedusha.
3) Finish tachanun.
4) You can sit for ashrai.