Surely the receiver would run plausibility checks on the received messages and reject spoofed locations that are physically impossible to receive by said receiver?
Wait until you hear about Sporadic-E or Aurora. RF is a weird place full of natural phenomena making the impossible very possible.
Yes, radio propagation is an entire academic field to be studied :)
In addition, if you have enough receivers you can use that to run something called MLAT [1] to also pick up GA aircraft that just have a transponder but no GPS. The more the merrier.
[1] https://adsbx.discourse.group/t/multilateration-mlat-how-it-...