A couple minor things here:
Tartan and C&C boats are all designed by Tim Jackett. I do not know Mr. Jacketts credentials in this area. A knowledgeable buyer should ask to see them in writing. A knowledgeable buyer should compare these credentials with those of other designers.
A lot of sailboat design is science, but a lot is still art. I don't know of any designer who has produced only 'winners'. As for credentials, what specifically would you look for? An academic degree? Membership in a naval architecture society? Perhaps these things can speak to the designer's knowledge of the science involved, but I don't know what they can say about the skill on the art side.
As for design and construction quality, a CE certification on the boat means it has passed European standards. While I grant you these are a pretty low hurdle, they're about the only objective standards out there for pleasure craft. If a boat is certified, the designer must be technically competent.
Beyond this, what other measures can you apply to the designer's credentials?
If the manufacturer switches an existing polyester construction set of molds to be used with epoxy construction with resulting thinner walls, there will be a lot of air space between these components when they are dry fit. A knowlegeable buyer should ask if the boat they are considering was designed to be manufactured in epoxy in the first place, with thinner panels, and that the molds were designed for these tolerances so that when components are dry fit, there are no air gaps.
I don't know what gaps you're talking about. I assume that since you're referring to the thinner hull skin, you mean gaps between the edges of the bulkheads (and other reinforcements such as stringers) and the hull skin.
If you're seeing gaps at these edges, it's actually a good thing. Bulkheads butted up against the hull create hard spots
. The hull skin flexes as wave pressures change. If a bulkhead is hard against the skin, the skin can't flex there. It has to bend along this line. This puts extra stress on the skin and can lead to cracking.
Better practice is to leave a gap at the edge of the bulkheads and have the tabbing bridge this gap.
Technically, there is absolutely no difference between molds used for polyester and epoxy composites. Whether a mold was originally used with polyester or epoxy resins will tell you nothing about the boat that comes out of it.
What you point out about the weights of the different boats is very interesting. Tartan/C&C pay for most of the hull materials by the pound, and epoxy ain't cheap, so they should have a very good idea of what their boat weighs. They really should have some explanation for the discrepancy you've found.
If you want to pursue this, have you thought about asking the FTC and Ohio Attorney General to investigate possible deceptive advertising? Tartan/C&C emphasizes performance in their marketing. Performance is a function of weight, so if they are understating their weights they are misleading customers.