The rifle in question has excess headspace. No doubt about it.
The fired primer is so far backed out that it won't go into a S/Holder.
With the 25-35, it has nothing to do with where the cartridge shoulder is.
The 25-35 headspaces on it's rim. Anything forward of that can be most any shape as long as it still will chamber.
That a light load shows the setback primer vividly and a full power load will make it go away, does not change the excess headspace issue of the rifle.
But,,Reloaders can fireform their case to the rifles w/slightly out of spec chamber to work around the issue.
Once the case is fully fireformed with a full load,,leave it that way. You have Zero headspace 'for your rifle'.
The case is now headspacing on the shoulder of the case and the breechface. Not just on the rim which the 25-35 does.
But from then on,, you must neck size the brass only. Do not push that shoulder back or you will again introduce more headspace clearance into the chamber/cartridge combination.
Repeatedly FL Sizing will lead to case head separation. That brass can only stretch so far.
To repair to issue, a new locking block can often be fitted to take up the slack. New/Old/Stock parts are getting hard to find. Sometimes a simple replacement with a used one from the parts pile will put the rifle back within spec.
It used to be popular to shim the breech face to make up the excess space. A steel shim soldered in place, then later epoxy was sometimes used.
I've seen some welded up and then nicely machined back down to meet spec. I've seen others badly mangled by welding too.
There's welding and then there's welding.
