Dew on the grass in the night/morning hours is plenty to cause an issue. Regardless of 24/7 or 12 hours overnight, that's not what the boots are designed for, as mentioned in the pamphlet that comes in the box with them. =) Though I'd still recommend them for it if you have a wound, I'd rather a little rubbing than an infected wound,so you have to pick which problem you can live with.
The vetwrap shouldn't have caused any swelling, unless it was too tight, then it could be a problem...
Another option is to just make a "duct tape boot" with gauze, vetwrap and an outer layer of duct tape, but it will have to be cut off every day, and reapplied.
By this point, it may be better to leave the hoof unwrapped anyways, once the abscess has opened, you can continue to treat it with iodine, but allowing it to dry out is the best thing. Depending on if was opened by your vet or on it's own. If it was cut open, the need to bandage is greater, as the abscess, if allowed to run it's own course, would have sealed itself from the internal blood supply, and the hole would just need cleaning,but cutting it open before it is ready to pop on it's own, can make the chance for reinfection much, much greater. However, even if cut open by your vet, after a week, the bandage is probably okay to come off, but keep cleaning it and applying the disinfectant for several more days, then you can stop that, as well.