When only one player is left in a simul, does he have to move at once or does he have 5 seconds per move?
The idea that there is a 5 second limit on thinking time in a simul is just nonsense. Early in the simul when the ordinary players have plenty of time while the simul giver is going round all the other players the player is expected to have his move ready and play immediately the simul giver arrives at the board.
That said, it is not unheard of for a player to "pass" or request a pass. Usually, in a well organized simul, there will be rules saying whether passes are allowed and if so how many (usually only one or two).
The simul giver is also not required to move immediately, particularly when it comes down to just 2 or 3 players. Believe it or not, he is often thinking about tricky positions in other games while he is making moves in the more routine positions and he is also affected by the cut down in numbers and the reduced thinking times.
Back in 1972 when I faced the late Viktor Korchnoi in a simul he rocked back and thought for a minute or two over one of my moves in the opening. He later accepted a draw which probably tells me a better player than me would have won my position ;-).
I played in an untimed simul against Bent Larsen in 1972. We had to have our move ready when he got to our board (out of some 20+ boards), with the option of one or two passes if necessary. I assume this could be decided upon differently in advance though by the exhibitor. Ultimately I was the last player left. I was probably the highest rated player of his opponents, although only Class A. At that point he sat down across from me and we continued at a relatively slow rate as I would have done in a normal non-timed game, taking perhaps two or three minutes per move. Naturally I lost, but at no point then did I feel rushed to make a move.
It is up to the organizer to decide on the rules, so anything is possible. A lot will depend on the level of players vs level of the exhibitor, so it is difficult to generalize. Most often simul exhibitions are fun events and rules are rather relaxed and participants cooperative.
In a typical simul players are expected to make their move promptly (e.g. within 5 seconds) when the exhibitors arrives at their board. If only few players are left it can make sense to give the players some extra time to think. For instance in this simul:
As there are no clocks, the opponents should make their moves in a reasonable time when few boards are active.
If necessary this could be enforced with a clock.
There are other ways to avoid this problem of few players and time:
- clock simul, where players and exhibitor get a certain amount of time
- open ended simul, where when a game finishes a new game is started, so the number of players is always the same