1 - No. He may take down the DTV Dish - I normally do but some guys won't - but the mast feet stay bolted into the roof. That's your seal. If you or someone you know wants to go up and take the mast feet out, they're 1/2" lags and maybe fill the holes with Black Jack, but I consult a roofing company. Dish won't do it because of the liability of damaging the roof. Plus DTV uses the same or similar Pitch Patch/Bishop's Tape that we use and that bonds to the asphalt after time, making it harder to separate the foot form the roof. The masts are 2 different sizes - ours is smaller - so the existing mast can't be used. Some subs/Retailers carry adapters, but they're not Dish Authorized.
Ask your installer about alternate mounts like corner or fascia mounts. A pole is possible, but Dish REALLY tries to get us to avoid poles at all costs - unless there's just no choice. It requires the Dish to sit on a temp mount for a week waiting for a locate service to mark the area and a return visit to permanently mount the pole and bury the cable. Also, poles are the easiest damaged. They get run into by people, lawn mowers, kids hang off them and water collects in the pole, rusting at the base from the inside out.
2 - There is no Dolby-vision and I'm not sure about HDR. In the years they've been out, I've installed 2. If you only have 1 4K TV, that's where the Hopper 3 should go. It's a 4K Receiver that does HDR and it'll save you $50.00 per 4K Joey. The Hopper is a system. The Hopper itself doesn't have to go at the "main" TV - other than aesthetics. But the Hopper is the Host - it needs to be cable in a distinct way. The Joeys do not and are only clients. Since the Hooper is what's doing everything at all the TV's and the Joeys are barely more than glorified computer monitors, they can run off splitters and have no real cabling requirements
3. The 4K Joey is only about 3/16' of an inch thick (and probably about 8" x 6")- it can go however you like and uses VHF remotes, like all Hoppers and Joeys do, so it can be hidden behind the TV, etc.. out of sight.