Definitely use the cabled adapter. Here is what I use (self made, silverplated copper in PTFE):
Second, you may wish to forgo using the balanced output, unless you have a very high impedance headphone that really needs 20V output.
The only ones I can think of would be directly driving STAX E-Stats via transformer "energiser" and the "Elephunked Ears" AKG K-1000:
Other than that, where even 20V may be on the low side), the single ended output will ALWAYS (not just iFi - all balanced Amp's) have less noise and less odd order distortion which causes a bright, forward and edgy sound if high enough to become audible than balanced. It may not always be readily audible, but objectively speaking BAL MUST be worse than SE, on any given Amp.
The one I did uses silverplated XLR on both headphone and adapter and the same cable. I tested this way back with the iCAN Pro 4.4mm, where I could switch directly between XLR4 and 4.4mm Penta - no audible difference with my adapter I could put my finger on.
Unlikely. At best you loose the final bit of "refinado".
This one, but don't overpay, it's just wires and plugs. Check the BOM cost and allow a sensible profit.
Thor