> I think that that suffix was never a "collective".
Never?
I do not know what it was in some Pre-Pre-PIE,
but in the attested material it has clearly the same functions as Semitic
"-at",
collective and feminine.
(Maybe the feminine marker originated in honorific use of collective marker,
similarly to the honorific use of pl. "you" instead of sg. "thou".
Or maybe the collective marker was originally used
speaking about a pregnant woman.)
P.A.