you can click on the little Info button with the question mark on the pack purchase screens to see exactly who is available in each pack too, but Captain Marvel still hasn’t even been added to packs yet so I wouldn’t expect Deadpool any time soon. There will probably be a pvp with him as the reward within the week though, like usual.
For season 2 or 3 I forget, I waited until they added daken and sentry (and I think mororo). Had to wait 3 weeks or so but I lucked out and got a daken cover from it. If you could use almost any 3* it’s probably not really worth waiting, but if you only need a handful of covers you might as well wait. Odds are pretty bad you’ll get anything useful anyway unless you need 2* covers, but might as well improve odds just a little by waiting.
It depends. I’m waiting, but it’s because over half of the covers I need are for the three characters that haven’t been added yet. But every new character added to the packs means the odds of getting a specific 3* goes down slightly, so, depending on where you are in your transition to 3*, it could actually be (slightly) counterproductive to wait. You’re also missing out on the ISO from selling most of your pulls.
Like most unpleasant things, you should do it as soon as you can and get it over with. Then the ISO you get from selling the nine 2s and your 6th green X-Force or yellow Invisible Woman can be used to raise one of your nearly maxed 3s almost one level. It’s better than waiting and then not getting one of even more covers you want. Minimize your disappointments.