Trying to choose a model

I've recently been looking for a good phone to join my parent's AT&T family plan. I've tried out the Note 4 and really love it, I just don't like the Samsung bloat. I want to buy a phone in about a month, and my budget is around 0. I searched on eBay for an unlocked Note 4 and found that there a lot of listings for AT&T unlocked phones at 0. At first, I though this would be great. It's not expensive, it's a great phone, etc. But then I looked at the XDA Developers forum and found a mess of different versions. After looking around, I'm pretty disappointed to find that the N910A is not rootable, which is a pretty big deal for me. I can deal with Samsung bloat if I can use a root disabler or something. But if it's not even rootable...

Anyway, I just want to ask a few questions regarding the Note 4 models.

1) Is the AT&T Version really not rootable? Is it just more difficult? (if yes, does anyone know why not?)
2) Would the T-Mobile version (N910T) work if I could find a nice unlocked one on eBay?
3) Will the pre-installed ROM update to Android M when it's available?

I hope these questions make sense.
Thanks in advance!