The £3 Corbynistas have the numbers if they become active in their local parties to have every one of the 172 MPs who voted against Corbyn deselected by the next general election.
This is the first chance they have had to get Corbyn out who despite the media narrative has lead the party to by-election wins and a good showing at the English council elections and Labour are doing ok in the polls (but Miliband did ok in polls as well...)
To deal with the second paragraph first, Labour did NOT do well at these council elections just gone. They lost seats. The best Labour can say is that they beat the apocalyptic predictions of losing several hundred seats.
The only party that did well at the council elections were the Lib Dems, and not to a huge extent.
Onto your first paragraph - the issue here is that there may simply not be enough time, and the media circus surrounding purges would be horrendous for Labour. Especially because quite a lot of those to-be-purged MPs are very popular local politicians - not all of them, mind, but quite a lot.
To de-select a Labour MP I think you'd need to do one of two things:
a) Somehow call an emergency (maybe it's called an extraordinary?) meeting of the local party, then table and pass a quorum vote. This would need Momentum members to have both the numbers and, I think, to have enough people elected to be in the local party's leadership team to actually call the meeting.
b) Have party HQ do it, either just de-selecting them as unsuitable candidates or kicking them outright out of the party.
The local party would then have to select a new PPC, and I'd suspect in many cases suddenly have to fill a lot of open spots in the campaigning team as those opposed to the de-selection would quit. That takes time, and we're looking at a general election by Christmas.