I'm no expert myself, but have done tiling - so here's my 2ps worth:-
Tiling on tiles works fine, and if the previous job was a good one,
makes the jobs quite easy. You could consider it a cop out, and I
would certainly want to remove both layers the next time, but then
again how often do you tile?

As for removing the tiles, I wouldn't worry too much about the plaster
unless deep chunks start coming off - tile adhesive can be put on quite
thick if required, not all over obviously, but in necessary places 1/2
inch is ok. More than this and you can level with mortar prior to
tiling.

I have only done this once, so I was a newbie last time, and found it
quite easy, with very pleasing results - so go for it!
Personally, I would remove all the tiles, then use a level batten to
tile onto, as removing the last row of old tiles might cause problems
with the newly laid first row.