Charger 2024 only applies to an attack taken as part of the Attack Action, AFAIK, which Flurry of Blows is not, so that's out.
I dunno how Crusher has changed for 2024, but the original version lets you move someone 5' 1/turn if you do Bludgeoning damage, so it depends on you doing Bludgeoning damage.
Open Hand Technique does grant a 15' push (STR save) to Flurry of Blows, regardless of usage.
Pushing Attack requires you to hit and then has a 15' (STR save).
So RAW this will really come down to the exact wording on Open Hand Technique, and what the DM sees as the "order of operations here", like, do these things go off sequentially - in which case you'd only get one of them - or simultaneously, in which case three would probably apply.
Re: edge of a roof - definitely - there are plenty of ways to shove people off things in D&D already and I don't think 2024 has changed the rules there.
Re: spiked wall - that'd be fine for Open Hand and Pushing Attack, but Crusher specifies you have to move them into an "unoccupied space" and I don't think many DMs would see a wall as an "unoccupied space", so unless the spikes extended into the square next to the wall, probably you couldn't use that unless you were able to convince the DM that the first 5' movement was from that, and the rest was from the others (which is some pretty hmmm rules-lawyering).