if you have removed the tone control, both sides, and still have the problem then it is not your tone control causing the problem. if you have removed the ground off of the 5 way, and still have the problem, it is not that as well.
we know the circuit works, but the problem lies in the effects loop when engaged. we also know that when the effects loop is engaged, the send side is working. so we need to focus on the receive side of the effects loop.
i need to see some pictures of the wiring.
Disclaimer: I make and sell Buffers. I also sometimes Modify, Build and sell preamps and amplifiers. My opinions are not intended to be sales pitches. I am a one man shop and prefer to spend time with my wife and family, i work full time on call and love to spend my spare time doing other things. I only make, modify or build things for those that seek them.