To my knowledge, units of different sides cannot board the same vehicle, which is why your east units disembark once the west unit boards. However, I'm not sure why neither side will get in afterward. In my experience, if one side is in already, the second side simply will not board, but your problem seems to be slightly different.
In any event, the only way I know how to work around this problem is to use the moveInCargo command. While the east units are already in the UAZ, use this command to move the pilot into the vehicle once he walks very close to it (maybe within 5 meters?). Obviously, there won't be a boarding animation, so it'll look a bit fake, but it'll work.
For your other question, I have a mission that's similar in that the players are supposed to rescue some POWs. What I did was have a trigger that checks how close the players are to each of the POWs. If any player is within 50 meters of any POW, then the POWs join the players' group. Obviously, this makes the POWs fully commandable by the group's leader, but that's what I wanted to happen in my mission.
So, you can do something similar. If you don't mind your hostage to be commandable, then have him join the player's group when the player moves close enough to the pilot. You probably want to make the pilot no longer a captive so that the east units will start firing at him, since they'd realize that he's escaping.