Those are for the IC chips’ pins, not for Dupont headers. The corresponding female connectors for those are https://www.banggood.com/10-Pair-40-Pin-2_54mm-Male-Female-SIL-Socket-Row-Strip-PCB-Connector-p-953436.html?cur_warehouse=CN&rmmds=buy. Alternatively you’ll have to use the IDC pin headers.
That’s a tough one.
Not that many compact phones right now in general. Sub-6" is a dying breed and anything sub-5.5" is almost gone. Best bet would be some older phones but they’ll all struggle with the modern web and android apps.
A solid camera narrows your choice down to Ubuntu Touch. Ubuntu Touch uses a compatibility layer for accessing Android drivers and firmware which PostmarketOS strongly discourages, so the absolute majority of pmOS ports have no working camera at all due to a lot of technical difficulties.
So for now look at what supported devices UBPorts has to offer.