Who is responsible for all of the labor, belts, and alternator and voltage
regulator check out?

The shop that did the work.

Should the mechanic who changed the belt initially bear some of the
financial responsibility for this?

Yes. However, you will never, ever be able to prove any of this, so I
don't see the shop doing anything.

This is just another example of the phenomenon I've noted here before,
where something else is broken by the act of fixing (or inspecting)
something else. It's happened every time I've EVER had any work done
behind the panel.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"