According to the OP, "Realistic" is:
1) The Normandy, which is near Earth's surface, somehow made it to the Mass Relay before the Crucible fired, knew to do so (even though 2 of the 3 endings would have been totally fine for it), and actually abandoned the most important battle in history at a critical point.
2) The Normandy, travelling at mass relay FTL speeds gets hit by an even faster "shockwave" full of energy, starts to violently shake and break up while at FTL speeds...and then somehow ends up on a completely habitable world mostly intact, with no serious injures. What.
3) The Synthesis ending: so this "wave of energy" changes DNA to INSTANTLY create huge amounts of glowing synthetic components inside the skin of organic people? And does nothing to synthetic life, despite being supposed to (EDI looks no different). And how does changing DNA make an organic being more synthetic? Without killing them, no less? How would it know HOW to do so for EVERY species in the galaxy, and every person?
4) The Catalyst controls the Reapers, but can't just kill them himself?
5) Why didn't the Catalyst just use the Synthesis option himself? He's had thousands of years to construct the Crucible, and knew how. All he needed was a single organic...
6) Why would using the Crucible destroy the relay network, yet leave the Citadel intact?
7) In the Destroy ending, how is all advanced technology destroyed without killing organics? Even some super-EMP wave would be lethal to humans if it's powerful enough to span huge chunks of the galaxy in seconds...