I know I have seen that done, but I don't see the benefit. And on the downside it would have made winding these transformers (and switching windings) a lot harder.
As for the second point, if balanced output doesn't give a benefit for short cable runs then what is your opinion of amps(Schiit Mjolnir1/2, Headamp GS-X, etc) that have a balanced topology and a balanced output? Should the balanced output only be used for long cable runs or in a professional setting?
Personally I just think that having an amp balanced all the way to the end has a sort of aesthetic appeal that you don't have with one that is balanced except for the last part. But that's just my bias.
I try not to have public opinions (positive or negative) of other people's amplifiers.
I will say that I use 4-pin connectors on amps all the time. I would prefer the whole headphone world move from TRS to 4-pin XLR,and where necessary simply ground 2 pins. So I agree with you on the aesthetic appeal. But here we are trying to keep costs as low as possible and appeal to as many people as possible, and everyone has a phone with a TRS jack. Maybe the T4 will have an XLR jack
I like differential for the power supply isolation aspect. Phones need so little power that getting extra voltage swing or slew rate it are generally non-issues.
You would never use 2 transformers.