To the OP
Sorry to hear of your engine oil pump issues
As you've already heard, this isn't something we've seen reported here before so sort of a rare issue.
Given the fact the Volt's ICE doesn't run all that often for most people, things like oil pumps should outlast the rest of the car
I looked (as the dealer likely did) and there isn't any sort of bulletin or anything related to the engine oil pump
In any case, I do agree your case might be a candidate for some goodwill and it appears Amber our on-site Customer Service rep wants to help. This is the exact sort of thing they specialize in and we have had LOTS of people get help from Chevrolet simply be letting the system work for them.
So please PM (private message) Amber with the information she requested and let her try to help you
You can do that following this link
As far as the top end noise, so much depends on how long the engine was run after oil pressure was lost. Since that variable exists and is based solely on your recollection, the shop would really have no choice but to replace the pump and reassess.
Since there were no apparent noises afterwards perhaps a piece of debris from the failure worked it's way somewhere that's created the new problem up top. But they will need to listen, scope, and isolate the noise which shouldn't be too hard.
Good luck and we look forward to hearing back from you on how things went for you.