The +- 7.4V should be good enough for most headphones.
Let's just take the HD650 as a random example. With a bit of headroom, the reproduced waveform should have a maximum of at least +- 6.5V, that's 9.2V RMS.
The HD650 is rated 103dB at 1V RMS, giving 122dB SPL at 9.2V RMS. That's ear shattering loud.
The HD650 has a rated nominal impedance of 300ohm, which gives us 282mW, this is in fact well above the maximum rated power of the HD650; 200mW.
I think loudness is more than adequate with rail voltages like that on most headphones.
Then the next concern is the quality of the power supply. Putting 6 batteries in series should reduce variation in the rail voltages. But as far as I know most batteries are already pretty darn smooth.
Additionally, and note that I'm no expert on this, a well designed circuit should be able to have a power supply rejection ratio big enough to make the variations in rail voltage that are already small, too small to be audible.
The final point is battery life. I personally think this is more important than that extra 3.4dB SPL. (20*log(11/7.4))
I have no idea how this is better or worse with the 3.7V battery. You'd have to fill me in on this. As well as charge time, but I imagine that option A and B charge faster due to the bigger voltages of the charger (USB voltage is low).
Something I almost forgot, but will option C use the same trick to double the rail supply as the Mini3? That is, an active ground channel? That was the biggest critique on the Mini3, and I would imagine it does indeed affect sound quality. If this is indeed the case, then option B may be a better choice after all, but note that I'm no expert on this.
Finally add in the huge convenience of option C's USB charging. I personally think this is an absolutely huge advantage that should not be underestimated. And especially the inability of option A to be used during charging is a very large minus. I think that it makes option A too inconvenient to be considered. Isn't it possible to change the circuit such that some of the DC adapter's power goes towards the audio circuit as well as charging?
All in all, I think option C is the best, but it does depend on how much sound quality is sacrificed.
Please point out any mistakes I made regarding circuit design, since I am only learning.