Very rarely do I comment on threads like these, as typically they don't tend to go anywhere. Nor do I consider myself a fan of Nitro, however they have taken great improvements to provide a value boat in recent years so hats off there.
However, builders clearly lay out warranty terms, any dealer should have set the expectation beforehand, despite if the issue is at fault of Nitro, or the owner, the warranty period on that component is expired, so a customer either has to get creative or purchase another. This isn't a Nitro problem, every boat builder that exists today wouldn't touch that issue, including BCB. Warranty is meant to protect the customer and give them reassurance in the immediate period after the purchase, not to protect all defects that it ever has.
Another example that pertains to this situation would be older Triton boats and their known defect to develop stress cracks, did the customer do anything to cause this? No, did the builder mean for this to happen? No, maybe there is a solution to when the boat was built, however regardless, unless these cracks showed up within the one year gel coat warranty provided, they were not covered.
Therefore, this is nothing unusual for Nitro, nor should it be reflected as bad against Nitro as they are following their rules they set in place when you decided to purchase.
In my opinion, I would recommend selling and getting into something different, as it seems that there won't be any improving with your Nitro situation. I would also look into the actual valuation of your boat, as it will be quite a bit lower than 51K and you should have the tax guy do the same.