Thanks for pointing that out. It's very reassuring, if it proves correct. But the letter you pointed to on Crackberry mentions a November security update. It's not specifically about Marshmallow.
In the FCC letter you received, can you please confirm for us that it speaks of Marshmallow in the same light, with an early November release?
That would be great. (There will still be the question about whether -- and how promptly -- VZ will continue to release security updates for Marshmallow going forward, but it's at least a start.)
Bottom-line lesson: Next time, buy my device from the manufacturer directly and use a GSM carrier.