by boomtron
I'm no expert, but I'm going with only one use per build action.The rules state that you can skip one space by paying the cost. Not one space per cost paid. Just like, for example, the Engineers can build one bridge as an action, not one bridge per cost paid.
Also, like you mentioned, the rules would certainly address if multiple tunnelings/carpet-flyings were possible in light of whether the end game adjacency scoring would take it into account.