A UI that might be relatively easy for the road problem: when you right click on an empty tile, in the left hand tile selector show all the tiles that would be

A UI that might be relatively easy for the road problem: when you right click on an empty tile, in the left hand tile selector show all the tiles that would be "valid" in that position. You'd have to add metadata about how tiles hook up, but you wouldn't need to get all fancy about constraints and stuff, or picking from several valid options.