There is in fact a previously attested verb of the ninkwis pattern seen in the +#PE22 'Common Eldarin' text, and this one postdates the emergence of the idea almost for fifteen years! The note in the PE17:76, the NGS bundle (1964), features tankarya, tankanse near tankata, tankante! This fits exactly to the described pattern which was ninkwisja, ninkwi-n-se. Though I expected ns to dissimilate into ss actually…