Can I use CAT 6a connectors with 7a cable (and get 6a performance)?

Generally speaking, if you mix "better" equipment with "lesser" one, your performance will be somewhere in between them, possibly even better than when using only lesser equipment: CAT6 cabling will not bump your speed up if you have 100 Mbit switches, but CAT7a cable could give you better EM shielding than CAT6a and thus reduce spurious network errors.

So, yes: there is no incompatibility between CAT7a cable and CAT6a connectors, and although you will not achieve CAT7a certification without using both of them (and connecting them accordingly to standards and best practices), you can safely mix them, it could even benefit your overall networking experience, and you'll be able to simply replace the connectors later without having to re-cable everything (assuming it won't be already time to jump to CAT9g cabling, of course).

Ultimately the 'Cat-level' is something that your cablers test and sign off against, I'd leave it to them to do using whatever parts and testing method they like, they're the ones that's sign and support it so just let them do their job knowing you'll kick them hard if things fail. If you were planning on doing it yourselves/internally I think you've either got a long job ahead of you or you're not going really get to a position of knowing it's Cat6/7 compliant sorry.

That said I wouldn't realistically expect the cabling to last to that level for more than 10 years, there's inherent longterm degradation of both connectors and cables that will increase attenuation over that period - even with most fibres. I'd bank on no more than 10 years to be honest.