Hi Eric,
Your questions are hard to answer without having the crank on the bench for inspection. That said, the rollers and the sleeves both look marked and worn, which for a roller big end is no good. You can verify this by measuring and close inspection.
The rollers can and should be replaced. If damaged, it might be possible to lightly regrind the outer surface of the of the sleeve on the pin and the inner surface of the conrod bearing (do the rollers run in the rod, or is there a pressed-in sleeve?) and fit oversize rollers, but replacement of the lot would be best.
Getting the correct end float for the big ends is part of the setup, although if the con rods are straight (this needs to be checked carefully) the exact end float is probably not critical.
I'm not sure what that shim is doing. But then, I would have said that the bearing cage could not possibly be original (quite the sparsest set of rollers I've seen in a big end), and the three pins... ghastly. A crank like this would be ok in a bike from the early 1920s, but it looks very out of place in the mid 1930s.
I'm not sure when pressed-up cranks came into common use, but my guess would be that a Velocette of the era would just have the crank pin pressed into the flywheels? Pressing the crank pin onto a shoulder is a solid way of doing things, but there is a problem if - as in the Douglas crank - there is a sleeve over the pin. If the pin has a shoulder, how long should the sleeve be? It is impossible to get both the sleeve and the shoulder to bear equally on the face of the flywheel/crank throw. I've recently done up and engine (a Spacke DeLuxe) with this problem (and more), but it was a 1913 design, not mid 1930s!
Good luck,
Leon