You are missing the point. Mewtwo and Darkrai will always give Genesect a SpA boost unless for some reason they choose to give it an Atk boost (aka investing in SpD). Sure you can Bug Buzz for the KO on Mewtwo and Darkrai and successfully revenge kill them, but you are looking at this scenario in a vacuum. If Terrakion revenge kills these two Pokemon with Stone Edge/Close Combat, a lot less things have free reign to set up. Of course, with a Scarf Pokemon you will always have these risks. +1 Scarfed Bug Buzz just begs for tons of Pokemon to have their way (most notably Ghost Arceus, Rayquaza, sub Ho-Oh, etc) with Genesect. I'd honestly choose to U-turn even if it can't manage the OHKO Darkrai/Mewtwo if the opponent has something like the aforementioned sweepers.
If your thinking that the majority of the metagame will put Def EVs just for Bug Buzz Genesect, you are dead wrong.
I don't see you're argument at all. Every single Scarfed Pokemon has to deal with the fact that something can set-up on them later, and I don't see any reason to discount Genesect as being good just because a few Pokemon can set-up on his Bug Buzz. Yeah, sure you can bring Ghost Arceus in on Bug Buzz, but you could do the same thing with Choice Scarfed Reshiram on Calm Mind Kyogre for example. For any Choice Scarfer, I could sit here and list off what could set-up on it since it's locked.
Also, you've already listed most of what can come-in and set-up if Gene Bug Buzzes; Ghost Arceus, Rayquaza (who easily dies to Gene's Ice Beam anyway) and Sub Ho-Oh; maybe the very rare Bulk Up Dialga? Honestly being locked in with a +1/+1 Genesect with Bug Buzz is not bad at all, and in most of my experience it's been nothing but a good thing (Never been set-up with being locked in either!) And I think you're also missing the point that Gene has a ton of other useful traits; being able to revenge a near-full Mewtwo at all is worth of notice because previously only a few Pokemon like Kyogre could do so.