Re: Replacing plaster wall
Sounds like the sheathing was applied to the inside of the studs, and then (maybe a good while later) plastered. Same as my 1850's house. If you wanted to, you could tear off the lath, add new spacers 16" on center to bring the new 1/2" drywall out to the same level as the original plasterwork, so the relative depth of window & door moldings and baseboards remained the same. If the spacers were 1/2" or more, it might be a good place to add foam insulation board.
Remove not the ancient landmark, which your fathers have set.