This is fairly solid except how they would be k-selected would they only have very rare hatching events? Then there is the matter of this species probably being most effective in the topics filling a niche that would struggle for variety and providing new forms of aquaculture. However I don't think that these shell-men are a going to be only wise old pacifists they would also be excellent merchants especially due to being able to access pearls, corals (dyes, and body scrubbers), and ambergris just to name a few. Another reason they would be excellent merchants is their long life span and if they can remember well enough, they can figure out oceanic pattern early and begin transoceanic trade routes earlier than other species or civs. Probably the hardest bit would be fine work like mechanical objects or gem cutting due to webbed extremities and they might also struggle with smelting iron before figuring out leverage tools to make them but cooper and bronze instruments would be doable and maybe preferable due to resistance to rusting.
Viewing post in Race suggestion: Shellfolk (Turtle People)
My initial thoughts as far as breeding goes is that since turtles take a huge amount of time to mature, an intelligent social species of turtle that lives even longer would likely invest a large portion of their energy into making sure their offspring reach adulthood. This would make breeding seasons quite sparse and spread out. Breeding grounds for feral shellfolk would likely come in the form of extremely isolated beaches in the far corners of the globe so as to better protect their temporary nests from predators, and all within a bale (that is actually the name of a group of turtles) would contribute to raising the hatchlings.
I really like your interpretation of turtlepeople as merchants, I could easily imagine shellfolk playing a niche similar to the Portuguese as the original explorers of the world, setting up the first global trade routes, and as the main brokers of most vital resources across the continents. Shellfolk hub cities would likely become extremely rich as a result, making both them and the trade routes prime targets for pirates and privateers. It would also likely lead to frequent trade wars between mercenary armies of hub cities as they attempt to monopolize trade from each region, akin to the British, Dutch, and French East India Companies (though they probably wouldn't be able to actually annex territory from natives, given their limitations on land).