Hmm.. The top image reminds me of the way our wall looked behind our saltwater aquarium, but I'm sure you'd have noticed if she had one. Could it be some sort of mildew is growing, where moisture got under the outer layer of plaster? (Note - I'm not necessarily talking about "deadly, get your family out of the house, toxic mold"). Is it possible that, as a growing organism, it would spread back under the surface - causing the separation? Efflorescence? I probably shouldn't even be offering (admittedly uneducated) opinions, lol. I rarely know what the heck I'm talking about! I hate that I'm always asking for help & advice, but never able to help anyone else. So I thought I should at least give it a shot. :-)

Depends on whether the keys are still holding it to the lathe. If not, Big Wally's Plaster Magic can be used to re-secure it, and you can skim coat over top of it.

Are you sure what you removed was plaster and not mortar? In either case, it's not really clear what you want to do. If you want to have the brick exposed and visible then you have a sizable job in front of you replacing the mortar by tuck pointing the joints- after removing any crumbling mortar bit by bit - where it has failed or been removed. This is quite a bit more than a "cosmetic repair". The mortar is what holds the chimney together. If you want to coat the entire surface with something there would seem to have been little point in removing what was already there. You would still need to do some fairly major repair work on the chimney before proceeding as well. Honestly and without wishing to be unduly alarming, the photos show a chimney in dire need of evaluation by a mason or other professional. By chipping off whatever was present, you may have seriously weakened the chimney. The mortar is in very poor condition and I would guess that there are many loose bricks. The expanding foam product shown in the bottom photo provides no strength and is unsuitable for repairing masonry. A coating of some sort and some paint is not going to fix this.

I cant help with the lime mixture, but, sometimes, in a house that old, with plaster that bad, it is better to remove it, and that way, can redo wiring etc that may be in the walls. You can also add more insulation that way too. If you are less than happy with the room, you did put sheetrock on, maybe have a crew come in and skim coat it, and add a bit of texture, that will look more like your original plaster.