Read the whole topic to understand the answer to your first question(or maybe just my first post in there)
And premium accounts double your credits and xp earnings from all matches as long as it stays active, so yes, it doubles your singleplayer rewards as well as it doubles multiplayer matches(both ladder matches and private matches).

So, what I do is join the 1 v cpu and the mech has about 150 health, I kill him in less than a minute and get 2,300 coins, then kill the next two the same way and collect and close out the star then find another etc... I make coins fast! And if I need say 2k exp, I can get it as well without totally going in the negative on my K/D . My K/D is horrendous so don't ask me on that, but I can farm coin and watch a movie, when I am done I can go do a lot of shopping and then cook my trash and upgrade my stuff! Sometimes I buy stuff just for trash to cook...

We're talking about the singleplayer battles AFTER the campaign, you have way to go berfore you actually can see what we're talking about.