Nava. Nava was atrocious in RF, which is harder to play in Fenway Park. He'd have a worse year than in 2013, which sounds almost impossible. He'd wouldn't be worth starting, even if he repeated his 2013 season with bat. You'd also be putting Gomes into LF. Nava and Gomes would be the worst corner outfield duo by a good margin. They'd also be forced to hit against pitchers they usually sit against. Gomes' numbers would collapse facing all those righties, and Nava, although not as dramatic would see a decline against more lefties. You also are moving the best defensive RF into CF, which is harder to play. Basically, you want Gomes in the lineup over Bradley Jr. Does Gomes hit that much better than Bradley to justify that move? You couldn't just look at their batting stats to decide that. You have to consider that Gomes is probably about average as a hitter in LF, but he's a pretty terrible defender. That's a below average regular. I don't know what you expect Bradley Jr. to do this year, but given his track record a .700 OPS isn't asking a lot. He probably can do better than that, but he'd be OK hitting that low. Why? Because most scouting reports claim he's a great defender in CF. CF is harder to play, so a great defender in CF is a very valuable asset to have. As long as he doesn't hit as poorly as he did last year, he's an OK player to start. If you want to look for an upgrade, you should probably find someone for Nava.
As for the catcher question, it's probably almost a zero percent chance that a 37 year old Pierzynski will better a .806 OPS Saltalamacchia provided. If Pierzynski is just average at C, I'd be OK with that. Ross is a backup catcher, so he probably won't impact the race. I guess he can catch Lester if they want to let him.