Maya, I'm probably too late to help with this (drat this stupid time difference!), and it sounds as though you have decided against these trousers, anyway. I just wanted to echo Marianne- like her, I did not have to size up, per se. I've always been between a 0 and a 2 in CM- a 2 if the item is trousers or something fitted over my thighs, and a 0 if it's something fitted at the waist, like a full or A-line skirt. Me ending up with the 2 in these trousers was completely normal for me.
Now, I've never met you in person, but from seeing your photos, I swear that there is NO WAY you would be a 6 in these trousers. I always had the impression you were the same size as me (though with slimmer thighs), so my guess would be the 2 for you, as well. Maybe the 4 because I know you've said you have issues with your (invisible, to me) stomach, but it would only be between those two. Not much help if you've decided not to go for them, but I wanted to chime in, anyway.
BTW, Devin- you look great in those!