I'd say it depends a bit on who you want to cover. When I opened my hoard I had Thanos as a bonus hero as he was about to leave the Latest token pool and I wanted to make sure I got enough covers for him, then once I was able to champ him I switched to Black Suit Spider-man as he was 4/4/4 and thus easily champable (and a good partner to OML and Phoenix). I don't know how interested you are in getting Daredevil though.
I'd rule out picking all Three latest though since that means you have no Control. I'd also rule out Widow since she works best with Black Bolt who you also do not have fully covered. Doc Ock... eh, he's out of Latest now so he will only be a PvE essential once in a blue moon. Black Bolt or Thanos would both be solid choices though.
How many pulls do you have in your hoard? The higher the number the less risk you have going with a Classic.
It's probably better to open the hoard first, then contacting CS about the swaps. You might be able to champ some of the ones you thought you had to swap after all...
260 sounds decent. I'm no statistician, but the average should be 39 fourstar covers, 13 in each color if it is a perfectly even distribution (which it most likely won't be, but the two Gambit covers you already have should give you a Little buffer).
On the other hand, statistically you should only get two fivestar bonus heroes, so the question is how far along you'll get with Thanos or Bolt as your bonus hero - heck, you might get two covers for the color that's already maxed. It might be better to just go with Gambit or Daredevil to make sure you get enough covers for them...
As long as you are reasonably certain you will be able to stock 300 pulls Before the next token rotation (and by all means it seems like you have a good chance of doing that), there's really no harm in starting to work on the hoard early.