As far as vendors and hair salons go, while Williams is the closest town to the South Rim, it is pretty small. The town of Flagstaff is only a little further away from the canyon, but has about five times the population and thus a much greater number and wider range of services and facilities. Flagstaff is also a college town (NAU) and so has an especially large number of food and hair salon places on the university side of town (I personally prefer Alley Cat’s downtown, but there are plenty of others). So you may want to include Flagstaff when searching for vendors and services.

Grandeur Point is a relatively undeveloped overlook on the rim to the west of Yavapai Point. You can not drive directly to Grandeur Point. The closest access is to park at Yavapai Point and walk along the rim trail (about a half mile). Note that Yavapai Point is one of the most popular overlooks and so parking can be tough during the day (especially during the summer). Another option is to park at the old Visitor Center (near Yavapai Lodge) and take the Rim trail from there to Grandeur Point, but that would be a much futher walk.

As far as weather goes, while much of Arizona can be blistering hot in the summer, the Grand Canyon, Flagstaff and Williams are up at 7,000 feet elevation on the Colorado Plateau and thus have very mild summers. Summer time highs in late July rarely get above the mid to upper 80s. But be aware that July and August are monsoon season in northern Arizona which means a high chance of heavy clouds and possibly brief rainstorms in the late afternoon. To be safe, any outdoor events this time of year should be planned for the morning or very early afternoon at the latest.

It sounds like you have already made reservations with El Tovar, if not, do so quickly as summer is the prime tourist season at the canyon and rooms (especially El Tovar) fill up quick. If guests will be staying at the canyon, you should either reserve some rooms for them or give them advance notice as soon as possible so they can make their own reservations.
References :
Arizona resident and frequent Grand Canyon visitor who grew up near the canyon (Flagstaff).

Have fun! That would be a great place to get married!