I think the point wasn't to destroy the Reapers (they could just ftl away from the explosion anyway) but to destroy the Citadel and the intelligence controlling the Reapers
No, the point was to destroy the Reaper fleet when it arrives and you get them all in one place but well I rewatched the ending of Arrival it doesn´t work probably. Seems that the explosion is slower than light speedbecause they have to show the bang. Seems that the contaiment field keeps the energy sphere together for a few seconds anyways.
Otherwise hit it with an FTL ship, the resulting nova should be faster than any Reaper ship trying to activate his drive. I haven´t seen any mini jumps at a moment´s notice from them so far even if destruction is imminent or when a fleet jumps in their rear. Ok Reapers aren´t really bright but you don´t need to be a genius to hit the gas, turn around and come back.
If you want to blow up the citadel, just send a big one through the conduit. No need to wreck surrounding relays.
I don´t think that the Catalyst has a backup. He should have but it seems he doesn´t. Why? Well the backup doesn´t go online when the crucible docks and changes the variables in a way that starkid thinks offering a way to blast away every Reaper in existence and itself is a good idea. That´s really big trojan. 