as for the minimum quantity, this is what Trading 212 replied to my inquiry:
We would like to kindly advise that different trading instruments can have different minimum trading quantities. Unfortunately, you wouldn’t be able to trade below that minimum.
I can confirm that the minimum/maximum trading quantity is set by Trading 212.
Personally, neither I see the point of this minimum quantity and it would be great if Trading 212 explained the purpose of that.
Secondly, I think it is unlikely that all offered stocks will be available as fractional given the way they are traded (over-the-counter). If I get it right, Trading 212 acts as a market maker for fractional shares and bears the risk that if you buy 0.1 share of XYZ, there will be someone who buys the remaining 0.9 share of XYZ as soon as possible. This is probably easy for popular instruments but not for the others.