1. If you want to be on AT&T network, order the AT&T sim. It doesn't matter if you buy unlocked or locked to AT&T. The only time you need unlocked is if you want to take a tmobile phone and use if on AT&T or vice versa. There is no need for unlocked if you are using the phone on the same carrier it is designed for.
2. If it weeks on AT&T, then it will work on ST. They use the same towers, network, etc.
3. Can't speak to this one, but may show H+ instead, indicating the HSPA+ network, which is what the iPhone is no calling 4G. Regardless of what it says, i believe both of those are capable of using that HSPA+ network.
4. LTE is not available on the straighttalk plan. There has to be special provisioning on the cell number account for LTE to work, and that won't happen on ST. but I wouldn't think that would cause you any problems either, it just won't work.
5. While I would argue the the 4S process isn't all that hard if you jailbreak and use tetherme (that's what I did, and it was a breeze), it is my understanding that it is easier with android phones. I believe there is an app you download to change the APN settings, and some phones have that feature built in. You might try the thread that tells how to get android phones on ST. I believe it's stickies at the top of the forum.