IMHO yes but hey this is Amazon and who knows what can happen?
Tread with caution and you are right in doing this and you may want to consider just dropping a case to seller support.
Seller support will not do anything with your account and will not trigger anything, they can only view things or refer it. It is just worth getting clarification (I say that tongue in cheek and rolling my eyes) as you also have to take what they say with a pinch of salt.
They will likely say they will look into it, provide an irrelevant link to seller university that leaves you none of the wiser but at least when the process happens you have a case on record that you can refer back to.
Without knowing the setup with the trademark, it will depend on whether this is held personally or held by a company. Depends then on whether you are buying the company or buying the trademark personally.
Trademark issues only generally arise when there is an infringement. You maybe just overthinking it. Once you own the trademark, I don't see any reason for contacting Amazon to say this has changed. If someone else was then going to do something to infringe it and you had to take issue, Amazon would then seek documentation from you to support the claim after the effect.
I have trademarks but they are owned by my company so it is an easy sell as if I sold my company the trademark still belongs to the company rather than me personally.
I think what is important as I stated earlier, the brand registry and the selling accounts are completely different and you don't have to have a selling account to be brand registered therefore whatever happens to your Uncle's account is absolutely irrelevant to the brand registry.