I don‘t know about the cracks but I will tell you what my insulation guy told me , and I verified it via internet research. That is original rock wool. There is nothing wrong with it and its a fireproof excellent insulation. He said its the best thing to leave it and insulate over it, preferably more rock wool. Just my .02 but do your research before removing it. Good luck about the cracks. My house is a 1910 for reference.

How urgent is this repair it's not urgent at all if you don't mind the roof collapsing and how do I fix it? Are you asking for a list of tools,material and step by step instructions or phone number of a carpenter?

Get some estimates from several reputable roofing companies.

That is kind of what I was getting at, Joseph. Odds are it's been this way for many years. With no visible issues on the outside, I'm wondering if I can just stabilize it somehow.

Get a contractor to look at it. That said, although I'm far from an expert and, if I were, diagnosis from a pic is hard, it looks to me like the rafters broke through a too flimsy beam that was on top of a very solid beam. When did they do this? If it happened in the course of building or repairing the roof, the problem may have been basically solved. If it happened later, presumably the roof would have settled somehow, creating problems elsewhere. Try to figure that out.

This old house recommends putting faceless insulation bats perpendicular to the boards on the floor. There's no need to remove the existing insulation, new stuff will just get dirty and dusty over decades, too.

I'd examine each point where a rafter meets the sill and look for issues above.

Leozn Syrat, what?