Hi Johanna,
I have had my concertina for one and a half month now, but I did not really have time to try it out before now
What you say is interesting, you used the V as a start position but evolved from it, does it mean that you do not have any "bellow support" now? I found that the support of the base on my knee -as recommended by Martyn- was quite a good support in itself and the V is not really natural to me. Then it is good news when everyone says it doesn't matter
But then, when playing as "comfortable to me", I still get pain in my hands, due to the tension in the little fingers I think... 
I support the left end of the concertina on my right knee. I no longer support the right end on anything. I've thought about some things I might be doing that make it easier for me to play that way, without getting pain in my little fingers:
First, I got the thumb straps to fit snugly around my thumbs. (When the straps are too loose, the thumb can't bear any of the weight of the concertina, so the little finger has to bear it all.) Actually, the thumb straps on my Jack are much too loose for my thumbs even when they're tightened all the way - I solved the problem by inserting some rolled-up pieces of soft fabric into the thumb loops. It looks somewhat strange, but it works.
Second, even after the thumb straps fit correctly, it took me some time to break the habit of "pinching" between my thumb and little finger, and instead allowing my thumb to bear some of the weight.
Third, as I open the bellows, I allow the right end to droop downward somewhat. Then, when I change bellows direction, I quickly move the right end upward so that the bellows is curving up. Then the right end is again moving downward as I close the bellows - so gravity is working with me on both the pull and the push. It's hard to describe, and I didn't even realize I was doing this until somebody pointed it out to me, so it must be a habit I developed without thinking about it. But it does seem to work for me.