Righty ho, it's good to have a bit more insight into the aim of this design.
Whilst I wouldn't necessarily agree that a dual rail (not necessarily linear) HAS to be cumbersome, it often is. If you're interested in portability, why not use batteries? A pair of 9V batteries NiMH followed by a beefy opamp buffer or a sijosae discrete buffer (detailed here http://tangentsoft.net/elec/vgrounds.html ) could be a valid choice as I'm sure you could find room in the studio to have spare sets of 9V batteries on charge (obviously investing in twice the necessary batteries to minimise downtime).
While it is possible to just use the mid point between two batteries as the VGND I am certainly not a fan of this as one battery will almost always discharge faster than the other, giving seriously uneven rails.
Your analysis of the Mini^3 is correct.
What sort of headphones are you expecting to power of this (something like DT-770m or easier I would expect), Are you sure you need the paralleled buffers on the output or is this just a 'nice to have'? Is there a set budget for this amp? How are you planning to build it (custom run of pcbs, or stripboard etc...)?
Have you considered using something like the O2 and just putting a Bal-SE converter in front of it?
Should be some food for thought there...
One more question: on which continent are you located?