1. we didn't take 28 but 93 from challis to Idaho Falls is easy.
2. The Mesa Falls road is doable. There are a couple of grades, but nothing really bad. Unless you don't like mountain roads or your truck needs brakes. There is a nice little campground - Warm River - just off 47 on your way to Mesa Falls. As an FYI - we only went from Ashton to Mesa Falls and did not drive the road beyond that.
3. We just drove 32 to 33 to 22 yesterday over Teton Pass. If you are willing to do this road (route 22), then the Mesa Falls road will be nothing. There are 10% grades both up and down. They are long grades. We have an F250 diesel and a 8700 lb. tt. We made it no problem. But, we have tow/haul mode and use transmission 'braking.' I would not recommend it without a controlled descent and excellent brakes.
If you get the chance, Redfish Lake (near Stanley) is beautiful. There is a lot of great boondocking in the area but if you like campgrounds, try Redfish Outlet.
We stayed at the Aspen Grove Golf and RV resort while in Ashton. Do not drink the water - it has the most awful taste. The sites are a little difficult, the wifi is hit or miss, but the views over the golf course are great.
Stanley Baking Co. has the best breakfast!