Well, it's no surprise that the 9.5X57 is not a ".35 Whelen necked up" as the 9.5X57 was designed in 1908 while the Whelen is said to have originated 1922.

The 7X57 goes back to 1892 and, as you noted (and we have discussed before), has an entirely different shoulder than the 9.5X57. That is why it is desirable to draw _X57 brass straight (or at least expand neck) prior to sizing in a 9.5X57 die. Shoulders of other 57mm cartridges tend to be not only narrower, but lower than the 9.5X57. The 9.5X57 die won't even touch the shoulder of many _X57 cases.

Shoulders of .35 Whelen (or of .30-'06) are higher and are pushed down by the 9.5X57 die, though they truly are slightly narrower.

The cartridge headspaces on that rather slight shoulder.

Many people do, however, 'fire form' _X57 in the MS and report satisfaction with the result.