Can you respond to קדושה if you are not already done with your own ש''ע? I heard that you can if you are past a certain point after שים שלום/שלום רב. I just want to know if that's true and, if so, exactly what point in the ש''ע must you have passed?
Yes, this is true. This refers to after you have said the first "yih'yu l'ratzon" and are now in middle of "elokai n'tzor". Different authorities disagree on how you may interrupt, so here's some sources.
This website says:
The newer editions of Siddur Tehilas Hashem have a chart of permitted/prohibited interruptions. It appears that most of the laws are the same as bein haperakim of shema, except for putting on talis and tefillin. It says that:
Yalkut Yosef vol.1 Hilchot Tefila pg. 176 he says that one should answer "Kadosh" and "Baruch" in the middle of Elokai Nesor as well as the first 5 Amenim of Kadish (the Ben Ish Hai holds that one may even answer Amen DeBerachot. However, Hacham Ovadia I believe writes in Yabia Omer 6:48:4 that one may not do so). However, if one didn't say the first "Yehi Rason" he may not stop for kedusha or kaddish (first 5 Amenim) because it is part of the Tefila.
However, if one already said the second he may answer any davar shebikdusha, including Baruch Hu UBaruch Shemo. (see Yechawe Daat 5:10)