2020-11-10 : If the target creature can't attack for any reason (such as being tapped), then it doesn't attack. If there's a cost associated with having it attack, the player isn't forced to pay that cost, so it doesn't have to attack in that case either.

2020-11-10 : If the target creature does attack, its controller still chooses which player or planeswalker it attacks, as normal. It can attack opponents who don't control untapped Trench Behemoths waiting to crush it, if any are available.

2020-11-10 : You can activate Trench Behemoth's first ability even if it's already untapped and/or already has hexproof.