I’d like to advocate for the devil, without getting feathers ruffled if possible. I think the issue of motivation is more primary than the technicalities of halachic debate in dealing with this issue. Before that sets off a blue touchpaper, let me clarify that I’m not casting aspersions on anyone supporting ‘PM’s, rather as follows:
I think it’s clear that what mostly gets referred to as metahalacha these days (or halachic values, hashkafa etc) is part and parcel of a posek’s approach to almost any question, especially where significant sociological/communal implications are involved. Trying to bring it down to purely a question of debating relevant technical sources isn’t realistic or accurate. I strongly suspect that anyone debating this has an essentially instinctive answer in mind, either for or against, and the debate is therefore skewed before it starts.
More fundamentally though, before even looking at sources, the starting point for such a question as ‘should there be PMs’ is what are we trying to achieve? A question which has no particular wider ramifications, like a technical issue in borer b’shabbat, just for example, can be treated directly as is on a technical basis. However with PMs, which have significant sociological ramifications impinging on a major feature of the last 2500 years of Jewish life, presumably there needs to be a clear and identifiable benefit in terms of the most fundamental Jewish values in order to proceed. This ‘motivation’ needs to be explicit. It can’t just be a question of ‘is it mutar?’.
Best example is probably the Beis Yaakov start up in the 20’s.The need was clear, explicit and involved the basic religious functioning of the next generation in Europe. It was radical, and the need justified it.
I’m not saying such a need doesn’t exist with regard to PMs but it’s clear that unless a ‘l’shem shamayim’ need can be identified clearly, it’s unlikely to win senior halachic support with technicalities being a secondary, although vital issue. I realise that many feel that contemporary women’s feeling of estrangement from religious involvement is a necessary motivator for this change, but in that case the argument should about that issue rather the technicalities of sources.
As a slight side point, I think you’re incorrect about your understanding of Bnot Tzelofchad. The midrash you quote makes clear that their question was debated and answered precisely because they were ‘wise and knowledgeable’. If their character had been otherwise (eg knowledgeable but not wise) then Moshe would not have engaged in the way he did. The nature of the person and, by clear implication, their motivation in asking the question is made central to the story.