Re: How do you prep plaster walls for ceramic tile?
my situation was similar; I had 1907 plaster/lath, with Abitibi board glued on the lower portion with dabs of construction adhesive 20 yrs. earlier, and wallpaper above. It tore all the accretions out, repaired the old plaster, and primed it with oil primer, then tiled over it to 42" ht. with subways. The upper portion of the wall got more significant plaster resurfacing since it got paint. A limited area like a bathroom is not too bad to get really smooth.
A modified thinset sticks perfectly well to fresh clean primer.
Remove not the ancient landmark, which your fathers have set.