Not sure where I got the impression it would be sung in English. Might have been something else they were promoting. Yes, Netrebko was marvelous...the tenor was ok, but not the best Nemorino I've ever heard. And, yes, it might just be that Adina didn't have much to pick from...the sergeant is certainly no prize!
As for why she fell for Nemorino? Well, Dulcomara's elixer is pretty potent stuff