Hi all. New to the forum. Please forgive the newbie questions.

I've been flying for many years, but have never been diligent about earning miles or keeping track of where they go. I want that to change, but I also value simplicity and thus want to strike a balance between number of mileage programs and earning potential. However, I'm not sure where to start, so I thought I'd ask here for help.

From my understanding, I should be able to get away with one mileage program on each of the three airline alliances (Star alliance, Oneworld, and Skyteam). My home airport was Detroit when growing up, so I have had a Delta Skymiles account for quite a while, and in the past it's been the only one with any consistent activity. However, I now live in the SF Bay Area, and often fly out of SFO and use United, so it seems like I should get a Star Alliance account. I typically go on one major international trip per year, and about two or three more domestic trips. Usually most flights are on United or Southwest out of SFO, and one flight is on Delta to my parents back in Detroit for the holidays.

Here are my questions:
1 - Should I get a United MileagePlus account, or an account with some other Star Alliance member even though I'm likely to fly on United the most? For example, I've heard that Alaska Airlines and US Airways have good mileage programs (though I don't know any details), so should I enroll with them instead and send my United mileage earnings to those programs?

2 - I have a US Airways account with 10k miles in it that just expired. Is it worth paying the $50 to reactivate it, perhaps so I can combine it with miles from my upcoming holiday United trip? I know there's also a pending merger that would switch US Airways to Oneworld. How should that affect the decision?

3 - What mileage programs should I use for other alliances?:
Skyteam: Even though I have had a Delta Skymiles account for a while, I recently redeemed a ticket and thus would be fine abandoning the account and opening another Skyteam partner account if it makes more sense.
Oneworld: Should I stick with my British Airways account? I never fly British Airways. Or should I open an American Airlines or other account?

4 - Should I get any credit cards? I significantly prefer 1.5%-2% cash back cards for my spending since I deeply prefer actual cash to arbitrary airline points, but I'm willing to open an airline credit card if it doesn't have an annual fee and will help me out in some way. For example, can I simply charge a dollar or two per year on these cards to keep my miles active?

Thanks!

Current miles:
United: Not enrolled
US airways: 10k expired
British Airways: 12k
Delta: 17k
Jetblue: 0
Southwest: 1k