how much of the nasty stuff which coats turbine blades with glass and blocks pitot tubes
For a quantitative assessment you need to evaluate all the effects.
On the available published evidence (and ONLY that - I'm not privy to confidential info from engine manufacturers), ash at low levels is quite unlikely to coat blades permanently or block pitot tubes. OK - particles might melt and stick temporarily onto turbine blades BUT chances are (speculating a bit here!) it seems UNLIKELY that there'd be enough to form a continuous film on the blade, given the conditions inside the engine. Then, when the turbine's internal temperatures cycle (due to throttle up/down and especially shutdowns and restarts on the ground), differential expansion movement of the metal and ash particles MAY allow it to flake off, causing no further problem.
A key issue with small quantities of ash, ESPECIALLY if there are repeated ash encounters, is presumably going to be build-up of accumulations in confined spaces, such as inside blades, in the air galleries serving the cooling holes. Unless blade cooling fails, this ash presumably would NOT melt but seems to me there must be plenty of possibilities for lumps of it to build up. Of course, if the air-flow IS impeded, then the blade overheats, the ash melts and the blockage becomes permanent and potentially very damaging.
What I've not seen yet is any comment about possibilities of removing ash post-flight BEFORE it builds up and melts.... Engine washing (eg. P&W Eco Power) is already in use for other reasons - maybe it would also be relevant for ash.
Over to you, engine experts.