I've seen a home like yours where the interior doors weren't seating against the stop molding. They doors were tight against the stops at the bottom bot not touching the stops at the tops of the doors. The house was about 15 yeas old (iirc).
I hadn't gone through the crawlspace yet, usually I go in the crawlspace last. In the attic there were metal ties, about 10" long strapped across the two halves of the structure. Effectively connecting the two halves together. The ties were screwed into the top of the plate of the two abutting walls. About half of the straps/ties had pulled loose and the top of the two abutting walls had moved apart about 1" at one end of the building. About 1/2" movement in the middle of the building and almost no movement at the other end. That's pretty significant movement as far as I'm concerned.
We found a crack in the middle of the foundation on the side of the home with the movement. The crack was fairly small, about 1/8" at the top of the crack and hairline at the bottom. All I could see was about 18" of the foundation above grade. I could sight along the sill plate in the crawlspace and see that it was bowed upward in the middle. I guess that was enough.
I don't remember seeing any issues with the siding but this was about 5 years ago so details like that didn't stay in my brain. I would assume there would have been some loose siding. Have you looked at the siding by sighting along the bottom for an upward bow? Or have you looked in the attic? A lot of times I miss foundation problems but because they get amplified on the roof or in the attic, I see the problems there... Just about all foundations have cracks so I don't get too worked up abut them unless I see it causing other problems.
They didn't buy the house. Not because of the problem but because the seller wouldn't cut the price of the home enough to get it fixed.
If the seller would have worked with them, the plan of action would have been:
1. Hire a structural engineer to evaluate it and make repair recommendations.
2. Get bids for repairs.
3. Ask the seller to pay for the engineer and repairs prior to closing or reduce the price of the house that amount.
By the way... failing roof trusses can also cause the two halves spread apart but I don't think you'd see siding problems on the sides of the house.
You will probably see the front and back exterior walls bowing outward where the trusses are failing. You might see a valley/sag along the roof line. This is a pretty easy fix. Pull the two halves back together with comealong winches and repair the trusses.
The house also had a new roof on the front and about a 30 year old roof in the back.
. Looked good from the street.
The way they fix these is to determine if the foundation is still moving. If it's not, They might just jack and shim up the exterior walls to level and re-seal the bottom plate at the foundation.