Further update: after some daytime exploration (made possible thanks to new enchanted gear with health, stamina, and magicka regeneration), I finally found a path.
It's a small, very hard-to-see trail that starts immediately to the SW of the shipwreck Orphan's Tear, and after some switchbacks, it leads past the broken tower with the Frost Troll onto the western-most cobbled road (the one going to the Thalmor Embassy and passing by Fort Hraggstad). You can connect to the main road from there. It's not far out of the way as I was going.
Alternatively, you can just clear out Fort Hraggstad and use that for a fast-travel point, or one of the other caves along the West Cobbled Road. (I don't know what else to call it!

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Edit: I've decided to adopt the tactic of VL flying from the Castle to Orphan's Tear, then hiking up the trail. Wish there was some way we could mark it with dark stones or something, it's hard to see, especially at night. I suppose I could leave cooking pots at various points, but no telling if they'll stay or if they'll bloat the save file. Serena's already done enough damage with her ash piles.