Yeah, they are definitely just fine rolling with the blanket as long as it is properly adjusted. When I keep a horse in pasture I try to leave them naked, lol, when possible, and always allow them some time to roll bare but they always end up rolling in their blankets and sheets, too. They do just fine.
I keep my belly straps somewhere around a full side hand (like with the top of my thumb on his belly and pinky horizontal to the ground and just resting against the strap) and hind straps around the center of the top back of his legs (the part between the bottom of his butt cheek and his hock)
I hope that makes sense, haha