Stepping back to answer kedusha while person behind hasn't finished praying

Question

According to Halachipdeia (source) you're not supposed to walk 4 amot in front of a person praying the amidah.

I think you also have certain restrictions regarding which parts of kedusha could be answered prior fully completing your Amidah (taking three steps back and then three steps forward), unless perhaps there's an earlier period where this restriction is removed (perhaps the second yihyu leratzon).

What should you do if the chazzan began the repetition but the person behind you is still reciting the amidah? Do you stay where you are and say the kedushah with whatever restrictions are imposed on you (such as, I've heard, not saying the first sentence of kedusha - nekadesh/nakdishach)? Or does saying the full kedusha take precedence over walking within the 4 amot of a person praying behind you?

Answer 1

The Mishna Berura (122 sk 4) says if you can't step back because of someone behind you but you have finished saying Shemone Esrei then you can answer even a regular Amen (not just the specific "top priority" responses permitted during, eg., Shema). He quotes some who say you can even answer "Baruch Hu uVaruch Shemo" (an even lower level requirement). It doesn't seem to me that saying the line "Nekadesh" is on that level (cf. OC 125) so it is not clear from here if it could be recited but CYLOR for a final ruling.

It seems clear from this discussion though that one should not break into someone else's Shechinosphere for these responses.

Answer 2

Late, but please note:

You NEVER have to take 3 steps back to answer ANYTHING. Upon completing the entire sentence, "Yihyu l'razton..." it is considered the same as if one already took the three steps back that immediately follow it. The point of both are that they are a positive indication that your t'fila is complete. You are therefore unrestricted from interruping with speech. See SH"A 122.