Re: 1898 Sitting Room with Exterior Walls to Bathroom
Keep all of the plaster. Scr-ape off all of the loose paint, etc. You will bet a far better result tiling to the old plaster than to new drywall. Have the electrician keep the holes for the new wiring to a minimum. Pull up all of the old flooring; save the finish floorboards for repairs elsewhere. Do not use cement board. Us a 3/4" layer of CDX (or BCX) plywood, and then a 3'8" layer, then apply Ditra mat and floor tile to that. Thinset does a great job of adhering to old plaster on its own, but you could use a bonding agent if there are areas where the white coat of finish plaster has peeled off.
Remove not the ancient landmark, which your fathers have set.