Yes, from what I know your way of measuring is definitely not standard. You can see that in the sellers pics, he is measuring the way Peacoat lays out in his fitting thread. The one you bought and returned does look to me like a true 42R from his pics.https://www.ebay.co.uk/itm/235839938503
I've measured a few of my coats and work jackets and I agree with you. When I measure P2P I measure from the seam that runs down the side of the garment from the armpit, which effectively is the halfway point. I suspect some people lay the item flat and then measure from the material start to the material end, which will add a couple of inches.
My advice would be to re-read Peacoats explanation of how to measure, then pick your best fitting suit or sportcoat and measure in the way he says. Don’t use an overcoat, since you will not be using it in the same way as an overcoat. Using those measurements, you should be able to find a coat that fits you well. You may need to have a tailor bring up the arm length some, but that is standard and inexpensive. I would not be very concerned about the length of the coat, there was a good deal of variance in how long those were cut back in the day (seen from viewing period photos).


