Can't tell you exactly on weight except to say that they're trivial - they're just molded foam. I can't justify buying a kitchen scale, however, to give you an exact, but I can have them with me next time I drop in.
I find that my feet stick to them surprisingly well; however, there is as noted little lateral support so if you try to step other than flat, your foot will rotate within the Waldie. As a person who pronates, I have to step carefully.
They don't absorb water, so drying is near-instant by just shaking them off. Contrast to my Tevas, which take at least a day.
Yes, the holes are easily big enough for a good-sized 'biner or strap.