IF withers is important, then the quest, or lack of one, is indeed odd. There's no good reason to go into that crypt that we're given.
Agreed, it feels less pressing if you rescue Shadowheart on the nautiloid. If you don’t, then you get the Explore the Ruins quest when you speak to her by the doors even if you don’t recruit her, though that may depend on what dialogue options you choose. I can’t recall when you get the quest otherwise, but at least if there’s a potential companion making a racket near the ruins it attracts attention gives you a good reason to approach. To be honest, I don’t know why you don’t find SH there even if you do free her from her pod. She could easily have landed in the spot near you, come round sooner, and wandered off.
I hope they don't pull another magic box at the right time situation, where withers just shows up in your camp one day if you don't go to the crypt.
Um, I think he might. Or at least I vaguely remember something to that effect, but I’ve never not gone and found him so may be imagining that.