Problem solved! I called my J.D. dealer Monday morning (Christmas Eve day) at 9am and spoke to the service department. The gentleman said they were short staffed due to the holiday, but had one delivery that morning at 11am and would have the technician come by afterward.
The tech arrived at noon and noticed the tension pulley looked high. He took it off, then noticed that one end of the bracket that held it on (it was "Z" shaped) was not level. The tech was able to bend it back into level and put everything back on in 25min.
After starting, we pulled the PTO switch numerous times & the belt held firmly in place and just in time, as we're supposed to get a big snowstorm here overnight.
I had heard good things about the Lakeland Equipment John Deere dealer (Avon, NY branch) I bought my X500 from, and it appears that they are as good as advertised! I didn't expect that level of service, especially on Christmas Eve.