So, thanks to everyone who replied. I just wanted to give you all an update.
After at least ten calls to Disney, including a few after I started this thread, this is what I learned:
If you have tickets from before a certain year (maybe 1995? 2000?), you canNOT link them to your My Disney Experience account until you physically bring the old tickets to the parks (which means no fastpasses set up in advance).
This past Sunday, a phone CM told me to contact
guest.services@disneyworld.com, because they would have the ability to help (if they so chose).
I sent off a brief, nice email with no trace of frustration, and concluded with the assurance that I understood if it really was impossible to set up the fastpasses in advance - I just wanted to the opportunity to talk with someone about it.
Within two days, a woman called me! Considering the amount of emails they must get, I was impressed and grateful.
The woman went into our
MDE account and added two fastpasses to each of my cousin's already-scheduled fastpasses.
We are thrilled! All of the plans we made revolving around those attractions and times can stand. I think what I'm most thrilled about, however, is the quick response time, and the fact that I feel listened to.
The email address was only offered when we were two weeks out; I wonder if this was the reason no one offered the email address before.
That being said, I think their official policy is still no fp+ with really old tickets until you're at Guest Relations in a park.