I'm adding a 30' snake cable to connect my home control room to do stereo miking in my living room. It's 6 a channel snake cable with no returns. What limitations should I consider in cable length, impedance, channel crosstalk etc if I want to use two of the channels for headphone return using only XLR / TRS converters / adapters etc. No disassembly or soldering.

Practically:
Scenario 1 (unbalanced): headphone out TRS>XLR ch6 feed into snake to XLR>TRS headphone output on the stage box in living room.
Scenario 2 (balanced): line out TRS>XLR feed into ch5 & ch6 (left and right) to XLR to TRS line out into headphone amp in living room.

Thoughts are welcome, and thank you in advance.

I would patch the two mics you want to use in channels 1 & 2 of your 6 channel snake cable.

I would then patch your return feeds to the upper channels 5 & 6, so scenario 2 would work best.

I do this all the time, and I'm running much longer cable runs.

Thank you remoteness!