1) I was looking in the files section here and that software for the MX-500 is there. But I think at this point in time, that's a deep rabbit hole that is going to lead to nowhere. I wouldn't recommend doing that.

2) I did some digging. The old MX-500 only has those 3 IR code sets for Sunfire. But in the PC programming software for RTI, URC, and others, there are a least 4+ IR code sets available. It could be that missing code set is the one that works your receiver.

3) Someone posted back in March that the NXG software didn't work for their MX-850. Not sure if it was because of their hardware (not using the correct USB to serial adapter or maybe the settings....). But it is something to consider before purchasing an MX-850. If you can't get the software to work, it becomes a paperweight.

4) I checked a URC MX-450 I have stored in a box and it does seem to have all the codes sets for Sunfire in it. The MX-450 is a modern version of your MX-500. I see 2 on eBay right now for $35 delivered. The MX-490 (the one URC told you to get) is the newest version that just came out. Not going to find those used yet.


Before you cause yourself to much of a headache, I would recommend grabbing one of those MX-450 from eBay. It's a more useful remote than the old MX-500 these days, with a modern button layout. You are already somewhat familiar with how it programs (and the on-screen makes it even easier). And at only $35, its a cheap way to see if it works or not.
If that doesn't work, then we will brain storm from there. Got a feeling it will work thou.