The slipperiness comes from the fact that the EU already admits that scanning of private messages didn't improve the catching and prosecution of perpetrators. Also, the biggest lobbyists for breaking E2EE argue that criminals are moving to encrypted platforms, and targeting encrypted platforms is actually the thing we need to finally put a dent in stopping the dissemination of CSAM

In plenty of cases it's absolutely self evident (especially when there are plenty of historical precedents for similar or analogous situations).

Yes you can get involved in extended arguments with people arguing in bad faith or whose world view is fundamentally incompatible with your but that's usually just a waste of time (I mean you wouldn't argue with Nazis either just the same as with people trying to institute a Stasi style surveillance apparatus through slow boiling)