There is a Living Trust, whose sole donor/trustee has Alzheimers and is no longer able to perform these functions. There is a paragraph that addresses who should become successor trustees should this situation arise, upon their written acceptance. My question is, should we have them present us with a Doctor's letter stating that this individual is incapacitated prior opening any account for this Trust?

Jo, we certainly would, even if there was no such requirement in the trust document.

Ditto ^^^. You need documentation that the trustee can't fill that role any longer.