1) A hit roll is any attack roll where you are trying to hit, be it ranged or melee. So they would re-roll both
2) Characters are indeed solo now, they can, however, embark on transports so long as they meet all requirements listed on the transport (it usually restricts to <ORDER> and non-jump units/terminators for Rhinos, all transports list their limitations and requirements). A step further on this reveals that now multiple units can embark on the same transport, and the "dedicated" part of dedicated transport isn't so "dedicated" anymore... a bit change from older editions.
3) Fortifications like the Battle Sanctum are specific units... and while your local group or TOs may allow proxy terrain for them... I'm sure they're going to want it to be of similar dimensions. The Battle Sanctum cannot have guns strapped to it, since it has no option to bring said weapons.
4) Yes. Further on this: some tournament/event organizers may enforce base size restrictions where they only allow the most recent kit's base size... so no 25mm terminators... or 25mm sisters. If you're looking to play in events, it would not be a waste of time to ensure you are on 32mm for your stock sisters.
5) Yes. A model loses wounds AFTER it fails all of its saves. Hit > Damage > Save > Damage roll (for multi damage weaponry) > "Feel no pain" rolls for each wound inflicted > deduct wounds and remove as necessary. It applies to all Valorous Heart units, including vehicles.
6) Dominions were the hardest hit on the conversion from old rules to new Codex. They cannot confer their scout rule to vehicles. What Dominions did gain, however, is a niche use for 4x Storm Bolters per squad to maximize "Blessed Bolts". They are definitely diminished, but still have a role in the army. Additionally, they can "Advance" before the game begins for some extra movement.
7) Valorous Heart + Imagifiers is pretty much the simplest synergy you can see. It has a lot of impact on the table. Bloody Rose Zephyrim are also really nice with their volume of attacks, Miracle Dice to land the charge from deep strike, and Tear It Down stratagem. There's a few other synergies, but those seem to be what's holding the army up and making the model range punch a bit above its weight.
8) They're noticeably different. A bit bigger and the style of the individual poses is wildly different from plastic to metal. Is it so much that you cannot mix the models? No. But I know I want to replace a large portion of my metal minis with plastic ones... they did a pretty good job with them.