My only double rifle with hammers is a Pedersoli .58 BP ML, but I grew up shooting side by side shotguns with hammers.

Carrying the gun loaded but uncocked, safer than a normal safety device, and similar I suppose to a modern decocking device.

Somewhat slower to cock both hammers for shooting but not a serious problem unless I suppose something big and nasty is a split second away from stomping you ...

"Second shot slower"? No reason if both hammers were cocked when shooting the first barrel.

I love shooting shotguns with hammers and am looking for a "new" vintage one to shoot clays with. Also if the price was right would not say no to a double rifle with hammers, say a .450 BPE (for medium game) or something similar or even a NE version.

As for swapping a .470 Merkel for a Jeffery .400? Well that is a personal decision based on any loss of $$$ value of the swap compared to original purchase price, which rifle will keep its value or improve its value in the mid or long term, what you will be hunting predominantly or intending to hunt (eg you mentioned elephant), pure personal preferences - which you like better? etc

Is the Jeffery .400 a NE proofed rifle?

If hunting elephant I personally would stick to the.470 without hammers.

If hunting Scandinavian moose or European boar (etc) a .400 might be fun to use.