Yeah, just judging from your sales thread you're a little bigger than I am. AFAIK Schneider sizing really depends on the type of garment. I only have knits and a jacket so my experience is limited, I guess.
^that wouldn't really make sense, the way boutiques work is they make a buy usually something like 6 months or one season ahead of time and that's pretty much it. best bet is to try and find a boutique that get's 3s (try Japan) or talk to schneider directly
The designer sets up a showroom/order form, stores make their orders, then the designer has the factory make that amount, and sends it off. So, special orders are probably not possible. There is usually no overstock so that designers don't lose money.