>Unpronounceable??
(a) Latin did not originally add - r to all verb forms! The -r- in the 2
singular is an accident, derived from the inherited -s-, going to -r- as
usual.
(b) Quite apart from the inherited *-dHwe or whatever, the attested 2
plural -te (later made clearer by the addition of an -s from the 2 singular,
and devloping to -tis) would not have caused any problems in pronunciation.
It might, however, have been too close in sound to the 3 singular -tur, but
there are many other cases of ambiguity, and it need not have caused a
complete rejection of that form of the paradigm.
But the real point might be that there never was an "add r" rule for the 2nd
person in early Latin.
Peter