If one target becomes illegal before Monoist Circuit-Feeder’s last ability resolves, the remaining legal target will still get +X/+0 or -0/-X, as appropriate.

The value of X is calculated only once, as Monoist Circuit-Feeder’s last ability resolves.

If you can’t choose both a creature you control and a creature an opponent controls as targets at the time Monoist Circuit-Feeder’s last ability is put onto the stack, the ability will be removed from the stack and none of its effects will occur.