Is a fully differential opamp subtractor possible?
This differential opamp computes a sum of differential signals, to turn it into a substractor simply swap the polarity of one of the differential inputs.
The opamp is a fully differential one like THS4131.
This assumes the source signals come from a low enough impedance to drive the resistors. If they are high impedance then you will need followers. This is the same issue as using a non-differential opamp in inverting mode, or as a substractor.
About the single opamp difference amplifier...

You can use it to turn a differential signal into single-ended, but if you add resistors you can also use it to add or substract differential signals. The output is single-ended, but... it is referenced to the node &quot;outn&quot;, which as the name does not iply, is an input which is set at ground here. You could set it at another voltage.
Output is the voltage difference between outp and outn, which is actually a differential signal... albeit without some of the advantages, since positive and negative halves aren't symmetrical, thus it has half the voltage headroom, and won't cancel some distortion harmonics.