Major physics conferences do not screen abstracts. Abstracts that are nonsensical are routinely accepted at certain very big conferences.
Small conferences may vary.
Physics conferences do not permit revision of abstracts.
Depends on the conference. Small conferences often accept most of things, at least to the poster stage.
In some fields, a conference is a place for researchers to meet, so conference papers have little significance.
In others, conference papers are almost as important as journal papers and its really hard to get an abstract in. Check ICCV acceptance rates....