I haven't heard about the Always & Forever Wedding Chapel, but the photos around the lake are beautiful! And it seems that you get a lot for your money whereas many of the hotels on the Strip give you random things in their packages (chocolate covered strawberries are nice, but I'd rather have more photos!)
August in Las Vegas is really hot, but at least it is a dry heat as opposed to a humid heat. If you have a shorter ceremony outdoors in the late afternoon or early evening it should be a little more comfortable for your guests. You could always leave a paper fan on their chairs to cool themselves during the ceremony.