Any is too much. Us a long straight bit of wood as a gauge and scrape
away at high spots with sandpaper and fill any low ones/

Bugger, oh well :-}

Would a splash of PVA over any fresh plaster be good enough to tile
onto ?
Not even that is needed., Just tile.

OK, wasn't sure, as I was asked in a specialist tile shop while we
were looking for some whether it would be on new or old plaster...

what's the score about tiling onto emulsion (just to brighten it up
in the meantime) - ok, or leave as-is (bare plaster) ?
Whatever.
Not sure why you are worried about it at all if its due to be ripped to
bits.

Because until such time as it gets done (which may even end up being
next year depending on what the as-yet-unknown-quantity of a plumber
(friend of a friend) who said it'd be a month before he could even
look at it, it's a bit dark and dismal in there :-}