What Do You Think is the Longest Length of Time You Should Wear Invisalign?

It certainly sounds like you have a lot of trays but I have seen longer cases.  I try to limit my cases to 30-35 trays as more means the case is very complicated.  Your dentists may be pushing the limits of what invisalign can do.  Speak to your dentist about your concerns. 

65 sets translates out to 130 weeks or 2 1/2 years....long but many patients have gone longer in braces which is much more likely to cause problems given that they are permanently attached to the teeth.

Assuming you have been checked and had x-rays taken to check your roots I see no problem with extended treatment.

As to the sensitivity....no idea why but I'm sure this has nothing to do with the time in treatment

Cases can be very long.  When they get as long as you are describe I usually defer to conventional brackets as they are much quicker and yield a better result.  I would step back and have additional consults or opinions.  It seems you may be trying to push the system past its limits.  Ultimately you can end up with an unstable result after all this time