I believe some bluecurve boxes have eithernet ports on the back and unless shaw disables them, they should be active., so I would prioritize getting the box working first over coax, then see if you can use the the Ethernet port to plug in a new or perhaps the existing wifi AP.
I'm surprised that this is all to relieve interference with your mouse, tbh unless it was a very expensive wireless mouse your simplest solution would be to get a new mouse. Older wireless mice operate on on the same band as older WiFi networks, so if your main router is not 5ghz enabled and your moca AP point was ( and assuming your parents have newer devices) , moving to a 5ghz network would help alleviate traffic for devices with newer hardware. Some wireless mice have a channel button on the receiver, so try switching that up, or even in your AP settings on the main wifi router you can change the channel as well (although again, this may affect the performance of the overall network for one device that isn't even on it).
Shaw also sells mesh access points compatible with their bluecurve portal, or you can buy your own, but again, this would not be your easiest or cheapest solution. If it's not a "special" mouse and you don't need it to be, it's easily replaceable. I can guess how much investing into a moca solution cost you, especially 7 years ago and it's a shame to see an incompatibility.
