I can't specifically answer your questions but I have a cat that has
had several episodes of not being able to pass the hairballs either
way. she has had to go into the vets for enemas until she goes.
since then I give her catalax or laxatone every day on her paw and she
has never gotten stuffed up again.
I would imagine that this could get pretty serious. My problem was
solved fairly easily.

Of course.  Endoscopy can be performed in the stomach, intestine (not much
of the small intestine), or colon.  I'd like to get my hands on a video
capsule endoscope for cats- its a small capsule that's swallowed and capable
of transmitting video images of the entire gastrointestinal tract.  The
drawbacks are can't get biopsy samples and it can't be used in cats with
suspected gastrointestional obstructions-- buts its great for everything
else-- eliminates a lot of exploratory surgery.

Can the stomach be

Most endoscopes have a biopsy and suction channel-- but most
gastrointestinal obstructions in cats occur in the small intestine-- which
is mostly inaccessible with an endoscope.  Ultrasounds are the best since
some materials that can cause obstructions are radiolucent and don't show up
on x-rays without contrast agents.