Rounds loaded with .357″ or .358″ bullets will fit the chamber with a couple thousandths to spare; the issue is the small throat diameter.
Loading and firing .357″ bullets in a .355″ bore is not a big deal; just start low and work up. Max loads are a little below the same for .355″ bullets, but less difference than some would expect.
On the 5.56 brass question – you can expand the necks out to 35 cal as has been done long before the Legend, and when fired in the 350 Legend they expand to fit the chamber just in front of the case web, looking about the same as fired brass from some of the looser hunting rifle chambers I’ve used. However, after trimming the brass to square up the case necks, the brass ends up about .050″ too short for the Legend chamber – no go, can’t use it that way.
again, Seems Winchester is underloading their factory stuff and the Hornady loads seems HOT. With this info I assumed ~2200fps as a safe max for the 180 PP. The Hodgen data lists Lil Gun starting 23g (2049fps) and max 25.5g (2213fps). I loaded up 24g (2257 fps), 24.5g (2288fps), and 25g (2359 fps). I only shot one round of 25g as I had the primer cratering around the firing pin and velocity was through the roof . All these loads appear to be over pressure by the velocity I was getting but there is little data out there.
I dropped back to the starting load of 23g and got velocities noted in the picture below. I have never loaded a cartridge so sensitive to seating depth both in terms of accuracy and velocity as is evident by the groups. .020 difference in seating depth made a good 20 fps difference in velocity with no other change, and seating this bullet out really opened up the groups. 2.125 OAL seems to be the magic seating depth for this bullet.