You wouldn't have to compete against yourself in single player, just like it is optional in multi-player (same for killing or sabotaging another character). If you control different characters there are different options, but you don't have to follow them all.
When entering the town in single player, it is also possible to just use the teleporter pyramids once part of the group is past the gate, if you fail to convince the guard to let the rest in.
For a possible AI, if companions can not be replaced, then as long as the game is designed and balanced for 4 characters, they can not permanently leave. They may object to certain actions or quest paths, etc, and there may be negative effects from having a low party moral, but in single player I don't know how feasible it would be to have a character leave, only to catch up to them again the next time you needed all four characters (since the main motivation everyone is working for is more important than the things they disagree about).
FWIW, in games where a party member does betray or leave the party, it has always resulted in me reloading the last save, so I could empty their inventory before that point. In Grandia 2, for example, a character leaves, and even though I was pretty sure he'd be back eventually, he had a couple pieces of unique equipment that gave special abilities in combat. I reloaded, swapped out the equipment just before I got back up to the point he left, and when he came back much later he had different equipment on (so the unique equipment may have been replaced, as well).
