If there is a Scarab generator in the works, like the Firefly Queen, then yes, the Scarab is fine as is.
Personally, I think that Immaterial should be a limited effect. Make it last three turns like the Dim. Shields do. That is the idea I like the most. I realize that I did not list it initially, I just missed it somehow, but that is the idea I would champion over lobotomizing them.
And, actually, you don't need to target something to cast a spell. As it has already been pointed out, Glories are targeted by animate weapon, so a spell that removes immortality could be done. I would probably have affect all creatures, and I like the idea of affecting permanents, but burrowers already suffer the attack penalty. The only reasons would be to get rid of those pesky Devourers, but otherwise the attack penalty is enough, I think.
And about Momentum + Quintessence; why not? Any Rainbow deck could make good use of it. It would be almost immorally cheap, but that's why I'm asking for a way to remove or limit Immaterial.
What about any of the other ideas? Scarabs and Immaterial aren't the only suggestions I've made. And what about you all? This is open suggestions, so feel free to suggest.