The customers buying C seats are buying C seats so it is hard to assume a fare premium for those. I was treating it as the 48 seats sold as all the C seats, with 8 OpUps since that's really what would be sold in the majority of cases. And since the customer would have a hard time guaranteeing that better seat it would be hard for them to commit to pay the extra cash for them. Might not be explained incredibly clearly, but that's what I'm getting at there.
You have some aircraft dedicated to certain destinations and those aircraft are configured to support the demand on those routes. UA has historically done this with the p.s. fleet. Other airlines have done it on a broader scale, whether VS with their smaller UC cabin for some destinations, AF with their CIO configuration or others. CO historically tried to avoid this sort of configuration due to the complexity of the scheduling, aircraft substitutions and such. UA has mostly avoided it, too, though the p.s. fleet was an obvious exception.

As the number of planes in the sub-fleet grows it becomes a bit easier to manage, but it is still added complexity.

I think that in the short term UA will keep F, with some specific destinations pulling that service from multiple hubs (BRU & ZRH are already getting that). The real question is whether they keep it long-term and to what extent.
No. And domestic service isn't really what's changing on June 1, at least not that I know of.