VC, I have heard that the 10.75x68 cartridge could at times suffer from some headspace issues due to the shallow case shoulder. Similar to the 400 Whelen cartridge, a heavy firing pin blow could allow the 10.75 case to move forward slightly in the chamber, enough to cause misfires with an indication on the primer of a light strike.
Again, like the Whelen cartridge, careful chambering and careful resizing of cases when reloading will help prevent misfires.
I have a supply of Berdan primed Kynoch 404 Jeffery ammunition from 1963 and have never experienced any issues when firing these cartridges. Perhaps your misfiring with your 10.75 Mauser is more to do with the chamber rather than the firing pin/spring or cartridges per se.

In reference to the RWS 400gr FMJ .423" bullet, when I obtained a supply of these from Australia for reloading my 404 back in the late 70's, they came with a short letter from the supplier warning of the issue of riveting with nickel jacketed bullets. However a simple test showed that the RWS bullets were in fact steel jacketed and while having thin sidewall jackets, they do have a good reinforced nose as can be seen in the image below of one I did section.
I shot a few Aussie buffalo with reloads using the RWS solids and they performed perfectly. The only bullet recovered from any buffalo was sandwiched in the rear hip ball and socket joint of a big bull after it had travelled full length from a frontal shot. The bullet had fishtailed at the rear of the bullet but the nose and jacket overall was intact.

RWS FMJ 401gr bullets, one sectioned showing reinforced nose.