Can anybody help me modifying a reverb circuit on a guitar amp to an FX loop circuit?
Point X thru a 1k resistor and 10uF cap would do for an effects send I suspect, and if you lift the end of C41 connected to IC6A, that would do as a point to connect the FX return. 

I would try that before doing anything more sophisticated.

The reverb mixing is done by IC5B, with the return muted by Q6 (when the gate is at ground), elsewhere there will be arrangements to pull the gate negative to enable the reverb doings. Point A,B switch the lead/rhythm channels in essentially the same way, these are mixed by IC5A. 

IC6B causes the reverb tank to be driven current mode, you could use it to buffer the FX send, but would need to change the resistor values.