Assume qeylIS writes an oath, which when heard, turns a coward to a warrior.

And this effect of the oath cannot be undone. It is a magical oath of sorts.

Could the former coward say:

muchoH rIntaH 'ip mu'mey
the words of the oath changed me

Or is the rIntaH to be used, only when the one who {rIntaH}s is
someone sentient ?

There is no requirement of sentience on the part of the subject.

jor rIntaH Hov
The star has exploded (and that's the end of that).

-- 
SuStel
http://trimboli.name