Best way to reverse a servo
All of the above ways will work, but perhaps you could just move the linkage to the opposite side of the servo so that your movement would be what you want.
I'd go with the extra channel and reverse it using your radio as my first choice. Why do you have to buy an 8 channel RX? I'm not familiar with the 8U, but can you use channel 5 and do a mix?
As far as the RX channel, either get a new crystal for yours or order another RX with a channel 52 crystal. You could replace the module on your 8U, but I think a crystal change would be cheaper.
Mike