Allowing bid units to blank territories or zones is a whole new frontier we don’t want to broach.
New Guinea is an exception that many players like, and allow, but if you have one exception, people will want more.
New Guinea has to be agreed by both players and should be done before bidding, otherwise you would probably need to bid over, yes.
I would suggest that players who want the New Guinea bid in play should be the ones stating that before bidding starts, because they are wanting something beyond default league rules.
In the event that you bid against such player and then find out he’s placing an infantry in New Guinea, you would then have to re-bid if you don’t accept it, and you can point out to him that it is not standard.
If you are in a playoff game and hypothetically the New Guinea player will not budge, he would forfeit the game because he refuses to play by league rules. Of course sporting and reasonable adults would work it out with zero animosity, but the answer is ultimately you have to play by the rules, this is how we prevent tug-of-wars
In regular season play, you would simply not start the game.