Is the speed of light the same everywhere in space?

[ImAnEngineer]

Yet another minor quibble, the speed of light is ASSUMED constant by SR, so it cannot be used as a justifcation of the constancy of c. You need to refer to Maxwells Equations to get that.

In my opinion, it can be used to show that c must be constant since the theoretical implications of the constancy of c are discussed in SR, and they turn out to be empirically correct. But it's not worth a discussion