Since I don't know what I'm talking about I probably ought to shut up ........ but that never stopped me before so ..... :rolleyes: On the ivory, if it is a 'tight' crack, I'd leave well enough alone and wait and see. Whatever stresses caused it may have been relieved and it may get no worse.
On the horn scaled Williams. I had a mammoth scaled Williams that began working it's way over to one side at the point until it finally was touching. I loosened the adjustable pivot bolt, moved the point to the center and tightened it up again. Once tight it was a bit right of center, but fine for opening and closing, and hasn't moved since. That's been months ago. Not sure if that would help, but worth a try.
On the heat/humidity issue ....... I had a crack develop in the back of a Gibson flat top with rosewood back and sides. I called Gibson in Montana to get a RA label. In speaking with the warranty dept the guy asked me what type of heat I used in the winter. I told him that me and the guitar have been in Florida for the past 10 years, and that I don't use heat at all.
He told me that they don't usually have 'problems' with guitars in that sort of environment. It is the instruments in the real cold climates that are exposed to the dehumidifying of heat sources drying the air out. Anyway .......... :gl: