jetsam
Minister of Fire
Good questions, and hard to answer.
1. I'm not sure that water is the main vehicle for mineral movement. Smoke (i.e. nanoparticles containing organic molecules - and more) likely contains much too. I would not be surprised if that carrier contains more minerals, as I think there is not much steam (small droplets of water) - it'll almost all be water vapor: given the relative humidity in the firebox the stability of steam will be negligible.
2. Catalyst poisoning is quite often related to compounds containing sulphur (and similar elements). Sulphur is a common "linker" of organic compounds to precious metal atoms (i.e. the active part of catalysts).
3. I am not convinced the acid bath removes minerals. After they have been there, at high T, a mild acid as vinegar won't really do much to minerals I think. See #2 for a possible reason why this works.
I do note that I am fairly knowledgeable in this area but would submit to people that actually have data, rather than the (well-informed) speculation above...
I can tell you that a vinegar bath definitely makes an immediate, noticeable improvement in a tired old cat. It's not like new again, but it's better. Doesn't seem to perk it up for too long, though. I suspect that wash coat flattening is a much bigger factor in performance decline than mineral depositation.