Am i permtited to stand SE behind a certain person when I know, or suspect, that this person will daven very quickly and then step back into my dalet amos and/ or sit down in front of me, which is assur? If for some reason the only place where I can daven is behind that person, is it better then to daven b’yechidus (later on) in order to avoid lifnei iver?
You should still daven in shul and not b’yichidus, and it isn’t lifnei iver. (As a side pointFirst of all you don’t know that it will happen as he may daven longer than you, or he might know the halacha). Nevertheless, try to stand a little to the side of the person and not directly in back of him, because then he can rely on the Arizal that it is only assur to cross directly in front of the mispalel. Even if not you should still daven in shul.
Chemdas Efrayim 14, Poskim.