Long post there, so I wont answer everything, but will comment on a couple of points
You're correct, if you use the percentage setting, the forum width does "adjust" itself to fit other resolution sizes. So members with lower screen res don't end up with a scroll bar. If you use a fixed px that's set to fit with a larger res, they do get a scroll bar and have to scroll left to right.
One thing I know from my own experience with the background image it this, if you use a single image, that fits your screen perfectly, it isn't resized along with the rest of the forum, so members with a different res will not be seeing exactly the same as you with regards to the background image. I tested it with my classic style forum using hubbys laptop after adding a fixed pic that I though fit fine, and as his is wider ended up with black bars down the sides.
The new style doesn't offer a % width setting at the moment, although we have asked Admin to see if this can be added, since it's very useful.
And you're correct, only owners, s.admin and admin have any control over the general appearance of a forum as they're the only ones with access to the forum control panel. As these settings affect the actual forum, not just your personal view of it, it's not possible to give individuals members control of this area, sorry.