Double, What fit are you referring to? Skinny Guy TTS. Wg I'd still at least down size 1. They will stretch but it'll take a while. My thighs on mine stretch some. Don't know how much but I don't wear them since I hate the fit
I would probably go 29. My deep indigos haven't really stretched much except for the waist with about 8 months total wear. They would fit a person with a 32" waist snuggly. So that's really not that much bigger than your 31.5 waist, plus you can wear a belt which is what I do. The thighs measure 11" so not sure how tight you want the thighs to be and since my thighs are around 18" they haven't stretched. You might be able to get them to stretch a little.
First, they are just jeans..so you wont mess anything up. That's the thing with raw denim, they all come out unique to it's wearer and what they do in them. You can wash, soak them when ever and how often you want. With my N&F I soaked them earlier than most would but I'm going to be washing/soaking these more than I'm going to with my APC's which I'm going to hold off for as long as possible.
But I suggest getting a pair of Naked & Famous even if you're paying...