1)There is a limud zchus. The Halacha is that if someone is not mechaven during the first brocha of shmoneh esrai his tefilla is void. Hence, say many poskim, if you see that the guy is looking around during SE & clearly not having kavana you can walk in front of him ’cause he’s not really davening anyway.
2)Talking during chazaras hashatz is THE ONLY PLACE IN SHULCHAN ARUCH where it says that one’s sin is “too great to bear”. (not to say that at other points in davening it’s OK)
3)Speaking during kaddish is l’havdil like speaking during the national anthem in the presence of the President. (not my mashal)
4)There is a famous story with R Moshe Ztz”l not walking in front of a child who was davening. I don’t have koach (or time) to type it out.
5)A table is generally speaking NOT sufficient as a mechitza because it needs to be 10 tefachim high by four tefachim wide with no gaps in the middle (which tables, shtenders & chairs usually have).
6)I have heard a Rav say that if someone is “metza’er” another jew by davening an abnormally long SE & thereby “locking” him in place – the entire SE is a “mitzva haba b’aveira”.