I've always liked these as well. I typically have a hard shell for any truck I have, and never can take it off or put it on as easily as I'd like, so I struggle through open bed jobs with a cap, then struggle through things that should be enclosed because I can't find enough neighbors to help me install it for beer. A friend has one on his late model Chevy and likes it, I've seen him pop it forward pretty quickly, and seems to keep things dry enough for him to not complain about it when we are out camping.
As far as your questions, from where I stand on the other side of the internet, I think I's scoot it forward a bit, that sems like a big gap there. Of course, I'd probably fold it down first to make sure that gap isn't needed for it to stack up when stowed away.
And as for the back, I'd default to thinking that the fabric would stretch ever so slightly in warmer weather, but that's just my gut and sounds like other's experience says otherwise. It might also be my viewing angle from online, but the gap between the tailgate and the bed side looks a bit large. Is it possible the bed sides have "splayed out" a bit over the years, so that the top bed rails are a little farther apart than spec? I had to take a come-along and correct my sister-in-law's Furd for that when the tailgate quit closing, and recently saw some ads for braces that are supposed to help that issue on the aluminum bed trucks, but a few too many round bales could do that to anything, I'd imagine.
Oh, and I've had clamps similar to that on my cappers for year without issues, even in a rear-end collision it was fine and held on. My biggest issue was always wear on them after so many removals, the aluminum threads would get a little weak. Eventually one or two got replaced with Harbor Freight C-clamps, which also held fine. One truck I did have some pin holes drilled, but they were more for alignment and shear, I never actually put nuts on there to hold it "down".