I believe they are not allowed on the opponent's court
This is correct; FIVB rule 10.1.2:
The ball that has crossed the net plane to the opponent's free zone
totally or partly through the external space, may be played back within
the team hits, provided that [...] the opponent's court is not touched by the player;
As to the interference point, this is best covered by entry 3.11.1 in the 2020 FIVB Casebook:
If [a player from team B1] moved within his own court, his play was legal.
On the other hand if he was in the free zone and his
movement can be considered as an interference, he
committed a fault.
This essentially gives three possibilities:
- The player from team B was entirely within their own court (the half of the court delimited by the boundary lines). In that case, they have priority and they can never interfere with an opponent.
- The player from team B was in the free zone (outside the boundary lines) but did not make an active movement. In the free zone, neither player has priority but as the player on team B did not make a movement, this is not interference2.
- The player from team B was in the free zone and did make an active movement. This is interference.
1. Team A and Team B are reversed in your example and that in the casebook - in the casebook, Team B is the receiving team. I have used your notation.
2. I'm looking for a better reference for this; it's what I was taught as a referee.