Homing and spindle control are only “connected” by the fact that they are both controlled by the grbl code running on the Arduino. Other than that they are separate functions and do not affect each other.
You need to have shielded wire for your homing switches, or a low pass filter on the switch inputs to the gShield.
It would also help reduce electrical noise if you used shielded wire for the spindle wire.
Use the Easel hardware setup function to activate homing. (there are other ways to do it but Easel is the easy way)
Check out this thread on how to use the switches.