So if reproofed,there would be both BP and NE marks......yes?
IN THEORY YES, Although sometimes the old BP Proof arks can be worn down or invisible. It will also have a REPROOF Mark stamp.

If original, just NE marks? YES, if originally Nitro Proofed and done at a time when Nitro Proof Marks were stamped.
Just remember that Nitro Proof guns exist that do not have Nitro Proof marks.

What sort of trouble arises with a reproofed gun? surely this is very dangerous.......??
Not always so - I am always a bit cautious but if it has been reproofed correctly which if done at one of the proof houses it would have been it should be OK.

Some BP were built like brick outhouses, especially when before they had worked out they didn't need the guns to be so big and heavy so some of these could easily be reproofed to Nitro (or Nitro for Black)

My concern is when people reproof a gun to increase it's value and that gun passes proof but you know that the action was never designed to take such a cartridge. It will pass proof but over time will shoot loose.
ie Cogswell & Harrison 375 2 1/2" NE Boxlock DR's - often seen around rechambered to 375 Flanged Magnum (the H&H Cartridge).
This action just wouldn't take the pressure and would shoot loose over a period of time. After all, why do Holland & Holland sell this cartridge and Purdey the 369 Purdey with a reinforce action design ? Because they worked out it needed it.

I have a small Webley style actioned Hammerless Boxlock in 360 2 7/16th BPE, This could easily be reproofed to NE except it would ruin the collector value.

Hope this helps.

500 Nitro