It appears there is some generalizing going on here between what a DP means on an Enfield pattern rifle, and what it means on a Ross Mk2 pattern rifle.

All of the Mk2 Ross' I've personally come across that were "DP" marked, simply had the firing pin end ground off. They were "DP" marked and sent to the cadet corp units to be used for Drill because the sights were graduated to the MK VI ball round (215gr Round nose), and not suitable for firing the Mk VII round.
Basically, they were marked "DP" because they were obsolete, not because they were unsafe. That said, what I did with mine, was have a Gunsmith look it over for any potential or real problems before I fired it.

You are correct in that the neck area is enlarged greatly compared to the rest of the chamber. Mine is too. It causes no problems at all, unless your brass is quite brittle. I bought some Win. commercial cases and have reloaded them 3 times so far without problems. I would however recommend neck sizing only, as the force required to full length size with that enlarged case is tremendous.