Header size and support for 10 foot window
A header of that size should be double 2x10, or at least 2x8. Sag is a concern (you don't want your windows to bind), but lateral stability is also. Walls like that tend to flex a lot in the wind and when doors are closed, creating pressure bursts. You don't want the thing bouncing and rattling the windows. You can see the flex in a 10' 2x6 just by leaning on it. That gives you an idea of what gravity will do over time.
Single trimmer studs are probably fine since you're not carrying overhead load. Double would be more appropriate for a header of that length if you have room.
There's really no point to joist hangers if you have supporting studs below. They're for carrying load, which there shouldn't be on the 2x12s unless they're intended to be beams themselves. I doubt they are. Face nail or toenail the header in wherever you can.
These suggestions are not gleaned from load tables or specific building codes, but from experience building in heavily-regulated jurisdictions. If you want to meet code (and not just achieve the intent of the code) you'll probably need to get someone on site.