Mike;
I'll take a shot at answering your questions;
First; The interior of your Mk I is all solid teak with the exceptions of veneer on the plywood bulk heads and floors. The aft cabin uses veneer bulkheads as well on the walls. I do not beleive that mahogany is used at any point, although, teak will naturally turn as dark as mahogany over time. 
  Given your current situation, I would try to remove the exsisting varnished areas and start over using a carefully applied application of varnish remover. Hopefully a PO (previous owner) did not use a poly- urathane because that will be more difficult to remove..
One of the beauties of teak is that it is an encapsulated (oil) wood and will lend itself to refinishing better than any other wood and will not absorb water. This is why it is used extensivly throughout the marine industry. Oil is encapsulated within the pores of the wood and over time, this oil will naturally turn a darker shade. People have used bleach and various 'Teak Cleaners' which usually contain some type of acid to attempt to lighten the teak and it is moderatly effective.
On our Mk I interior, which has never had any other finish beyond an oil, we use an application of 'Liquid Gold' twice a year. Exterior teak gets a yearly coating of Cetol and is canvas covered from the harsh Florida sun's UV rays. 
Personaly, I think it would be a mistake to paint over the exsisting teak with a paint since I like the contrast between dark and light and once on - would be problematic to remove.
Now that being said, why not try and sand the finish off a small area of teak and see if you like it ?
Your question: On the Mark II boats that came from the factory with a lighter interior, they are using a different wood for interior finishing other than teak. I beleive it is ash which is naturally lighter and comes with a pre-applied finish.
 Hope that helps.
Bill

Mike;
 There is another thought for the shelves that are showing signs of wear or water damage due to things like leaky chain plates -and that is to recover them with a teak venner  or a very thin piece of teak and glue it over the damaged shelf. We're fortunate to have a great teak supply place near by but a good wood worker or cabinet shop might be able  to direct  you where a good wood supply place is located in your area. Plus there is a good supply of teak on line. 
 As far as the solid teak pieces,just sand them down and start over.You can PM me if you need help.
Bill

Mike. On my Mark I, those shelves are a teak verneer plywood and are quite thin. 
Bill