I can probably only speak to up to 10 years, but here goes...
What has been your experience with 10-15 year old MC connectors that have not been disconnected since installation? I'm wondering how easy they are to disconnect,
Sometimes moderately difficult, even with the right tool, but I never had to give up on one.
do the seals harden over time,
Yes, a bit, I guess? But the ones I've seen were still serviceable. Some seals are pretty stiff when new, too.
and if reconnected do they still have a good seal?
I think so?
How well will an old MC connector mate with a new one?
I think it will do fine. It might be moderately more difficult to plug an old positive plug into a new negative plug than vice versa. Because the seal is on the positive plug, and if the seal has hardened ...
My experience was mostly with micro-inverters, but also some string systems and optimizers.