1) The pin 18 which I suggested is just a suggestion — it’s the GPIO that’s next to pin 15 is not being used. But you can use any GPIO pin that’s not currently used by OSPi.
2) I hate to say this but RPi makes a lot of changes and it’s a bit hard to keep up with.
As I said, since pin 15 works (e.g. you can hear the relay noise), why not just use that pin. The fact that relay is connected to that pin shouldn’t matter with the transmitter. You can always desolder the relay if you can (although it’s a bit tricky since a relay has 6 pins).