In other words, your argument is:
1. Inner quantifiers are incompatible with Substance. (True.) 2. {lei/loi PA} is grammatical and presumably meaningful. 3. Therefore, {lei/loi} mustn't be Substance. 4. This leaves the way open for {lei/loi} to just do Collective, as per standard piano-carrying examples.
That argument seems pretty unassailable to me, and I will join you in it.
I would change 2. to "{lei/loi PA} (especially {lei PA} in fact) is grammatical and used often, including in paradigmatic examples like {lei ci nanmu cu bevri le pipno}". If it were just that it was grammaticl but never used, then it might eventually be Substance, but in fact it is used.