Those heroes will be added to overwatch 1 too... So yeah
Heroes have to be available. As multiplayer pool is common of both heroes.
Pretty much, and models are getting a minor refurbish and polish. There's talk of a new engine but don't know if it will use PvP mode as there is crossplay between new and old Overwatch.
You should give paladins a try. I played it during the early beta/alpha and was pretty fun. Some forum members like ztr, jas and psyborg still play it from time to time
New engine could mess with physics, which could cause a problem in cross play. That is also the reason why CSGO hasn't moved to Source 2 yet. Source 2's new physics system would mean grenade trajectories could get affected and players will have to relearn grenade arcs.