For the earphones the polarity is not significant, so reversing the connections within the adapter will not matter, but strictly tip of 0.25" connector should connect to pin1 of the XLR.
I got confused. I thought the 0.25" connector
IS the earphones plug and on one end you say that reversing the polarity doesn't matter and on the other hand you say I MUST connect the tip of the 0.25" connector (which is the earphones connector) to the XLR pin 1.
Maybe I was not clear myself so I will re-explain the situation: I have a GA-type headset (with 2 plugs - one for audio [the 0.25" plug] and one for mic [the 0.206" plug]) which I want to connect to an Airbus/ATR plane (which uses the XLR 5 connection), and I also have an adapter for this purpose.
I reverse engineered the adapter to check which part of my GA headset would connect to which XLR pin on the plane, and I found out that my adapter will connect the TIP of my headset's audio plug (which is the 0.25" plug) to the plane's XLR pin #2 (which is in opposed to what you wrote that it must be connected to the XLR pin #1) and the SLEEVE of my headset's audio plug (still 0.25") to the plane's XLR pin # 1.
I'm attaching a picture of the diagram I drew while reverse engineering it to make my current situation more clear. On the picture there are 2 different mic (0.206") plugs, it's because I have 2 headsets which have a slightly different mic plugs. Disregard it.
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https://www.dropbox.com/s/mfyor49n97...%2009%2004.jpg)