can any stereo receiver be turned to mono?

Relax, it's a receiver. A $30-60 receiver, at that. Well worth trying for entertainment value alone. Or think of it as a mercy killing, like the way they do the injured horse. Only in this case afterwards while you air out the room you go on CraigsList and find half a dozen better ones listed under Free Stuff. After a quick search you go pick up the closest one, it will be sitting there on the curb, and maybe stop by get another one while you're out driving around. Then back home compare and use the least bad sounding. Just be careful not to pick up an AVR by mistake. 

Even easier, connect the + from both channels into one. +L and +R both go to chassis ground, so connecting L+R positive sums them yielding mono. This is what happens inside the Y connector. Either that, or the amp goes up in smoke. Which this being a receiver and knowing how I feel about receivers just may be my ulterior motive.

Only one way to find out for sure. 😂🤣