Can I cook scalloped potatoes for 1 hour, drive one hour and then cook for the last half hour at a relatives house?

agreed. I would cook as much of it as possible at home and then let it finish or reheat at the next destination.

Yes, however, it may need (considerably) longer than half an hour at your relatives house. Personally, if you have the time before hand, cook it the full time at home and then heat it up when you get to your relative's house. But if you don't have time to do it, I don't see why not, just keep a close eye on it.

Given your timing an you make it in the slowcooker instead?