One advantage of going to the softer side, it will give you a better chance, a wider window if you will, to obturate if your throat seal is not perfect. Lyman #2 is around 15, so the throat must seal, going down to 10-12 the powder charge may make it seal even if it isn't a perfect fit. Food for thought.
I notice Accurate molds has 35-180C pattern for the .35-.351 SL's. I once considered the .351 for a Martini Cadet conversion, but have more ideas than money.
True, mostly. To realize that phenomenon you gotta spank the bullet with a faster burning powder. Using a slow burner, which may well neatly yield desired velocity with a longer pressure curve, will start the bullet on its journey while pushing hot gasses up over and around it for a while before the bullet finally obturates, thus causing leading and probable quick fall off in accuracy. (Obviously ameliorated though by achieving proper bullet fit from the git-go.)
.351 WSL would be cool in a small Martini. Big hurdle though would be engineering the rimless extractor, not impossible but tricky. Why not just go straight to the .357 Magnum or .357 Maxi?
A local rifle loony buddy recently scored a pristine M1905 in .35WSL. Really nifty rifle. We're just starting down the rabbit hole to feed it. Neat/big deal with his rifle: it came with a spare 5-shot magazine! Those things are about as valuable as the guns themselves!
