Good questions ... and answers!
I sometimes wondered if maybe Jack had already had this motel room booked before he went to see Ennis at his place.
First off, he showed up kind of late in the day. If they were going to be out "drinkin' & talkin' and all"
then Jack would have needed an overnight place anyway, no matter how the reunion went.
He sure seemed surprised as hell when Ennis grabbed him and dragged him into the stairwell. But since he admitted later he knew they'd "get into this again", I wondered too if maybe he had figured on having to seduce Ennis. Probably thought he'd have to ease Ennis into the idea of being lovers again.
Shows what he knew! Ennis made seduction a pretty moot point, did a little himself, I'd say. Which would have made having the motel room pre-booked a very handy thing and make it a little safer for them both to sneak in later, too (?).