Returning player - what to do?

You ask many questions, I have many opinions

First, your plan to play PVP to as high a level as you can stomach is a good one, but I would suggest prioritizing PVE progressions rewards. You do not have to play optimally, since the game has changed the reward balance from placement to progression, but you should plan on getting max progression as often as you can manage to. For your trouble you'll get 3 3*, a 4*, 33 CP + 2 CP/day and assorted ISO and HP (assuming SCL 8; SCL 7 is slightly less and the way you describe your roster you can manage this). This is the main driver for building a roster in the current game. 

For hoarding, there are 2 major reasons people hoard, and I don't think any of them apply to you at the moment. It generally refers to not spending CP or tokens for some period of time for some reason:

• You don't have sufficient resources if you get lucky on a token pull. For example, you might not have HP to roster a new character, or you might not have enough ISO to champ a character. So, you hold off using these resources until you have what you need. Once you do, you start pulling the tokens until you're in that same boat again.
• OR, you've made a decision that you want to play in 5* land. I haven't done this, so others can give a better description, but you wait until the Latest Legends both have 3 good 5* and that you have the resources for 300-400 pulls (between LT and CP). By doing this, you'll hopefully get the covers you need to champ those 3 5* characters all in one swoop. Customer Support has been known to swap covers for latest legends (so, if you drew 14 covers for Spidey, but they were 8/3/4, they would make it 6/3/5 for you so you can champ), but this is not official and they could stop doing it at any time

For Bonus heroes, I'm not the one to ask. I have all 3* champed, so I'm using BH to rush characters to 270. Strange for 3* BH seems pretty reasonable until he's champed; in your boat, I would choose a highly rated 4* like Carol, Peggy, Gamora, Rocket, etc. for the same reason. In 4* land, I use BH to help champ; that's how I champed Peggy, and I'm working on Carnage now (for no reason other than I just want to run the Medusa/Carnage/Mr. F team). 

Finally, you asked about SCL. For PVP, SCL doesn't matter, but make sure that the first thing you sign up for in any season is SCL 8, since this will set your rewards for the season (and the SCL8 rewards are better). Your opponent pool is the same regardless of SCL, although the group you compare your score against is only the group in the same SCL in the same event. I just always run SCL8 and I can always make 575 or more, which is all I care about. 
For PVE, it's whatever you can clear quickly. If SCL8 takes too much time, drop to SCL7; the rewards are almost the same. Even SCL6. Below that, I wouldn't do, but as above, I don't think it's worth the time to try for the top progression rewards; others on the boards feel differently about this.

I hope this helps; beyond that you've hit the basics... Farm the 2*, DDQ every day, Lightning rounds, etc. Just have fun with it.