Yes its possible to play as the probe droid, just assign the msh to the character you want it to work for.

you could probably stand in a LAAT/i. I have not had experience with standing in flying vehicles, but I know that the uta_fly_gunship has an open space for standing, so you could probably use t hat one.

If you're asking whether the Prode Droid would work, I know xwingguy had included it his Battle of Endor WIP, though as a local team. So, yes, it's possible. As for setting up the unit as playable, that can be done just like any other unit. You should only need to add it to the lua.

The only major problem I know of with standing in the LAAT would be that the momentum of the vehicle would not transfer over to the unit standing inside. I keep thinking there's a way to display a person standing inside while taking a passenger slot, but I'm really not sure about that.

For #2, unless you are actually a passenger, on the LAAT, no, you will fall out, once the ship starts to fly, if you just walked on.

This however *Could* be fixed, if the vehicle's cabin was given a stronger Collision Geometry.