I think it's a fun theory; it fits a few things.
- Gustavo was a smart man who "groomed" people to play roles in his organization; like Gale who he paid to get a chemistry degree. Why not pay for one of his own sons to become a lawyer?
- Ernesto / Gustavo the obvious thing here; Ernesto is clearly a black man, but has a spanish name. Gustavo + black woman = child with spanish first name. (or Portuguese or whatever, not entirely sure the etymology)
- Saul says to Walt before getting him hooked up with Gus "I know a guy who knows a guy", and while it's implied that guy is likely Mike, I don't think Mike would actually ever recommend Gustavo deal with Saul. Mike hates Saul; but Ernesto on the other hand might suggest Jimmy/Saul as an option at some point for Gustavo if he ever needed legal help.
- Oh and look at this lighting used on Ernesto in the last episode:
FACE.... OFF