In-breeding should not be an issue. The Trolls from Djunsland are said to be the descendants of one couple - Picknose and Eddbit - during the 10,000 years of waiting. Unlessit is a monestiv society and all of them are literally born to Picknose and Oddbit there must have been a lot of inbreeding. And they are physically and mentally well and inventive. A social degeneration is more probable indeed ... tho I wonder what would have caused the former Forest Trolls and/or Ice Trolls to change so much. Wasn't there a discussion that it might be the influence of the pond of foul magic in the Holt area?