Won't quibble much over that highlighted, but will point out no material object can move at c relative to any other material object (excluding dubious inferences re cosmic redshift). Only asymptotically approach c, and practically, that will almost exclusively apply only to charged particles typically either cosmic rays or those generated in terrestrial particle accelerators.
No argument with the intent of what you say now. The problem was in earlier stating SR is ONLY concerned with such things i.e observation or merely theoretical calculation of kinematic effects.
There is far more to SR and it's application to many disciplines, from GPS to particle smashers to certain aspects of atomic spectra to practically all of EM. Which are all about interactions effected by relativistic motions. Designating observer vs observed is either completely irrelevant or arbitrary in such situations.
My point in my post # 342 and #559 is true .
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