Hello, all I have to do is stop at "block sealant".
I'm sorry but trying to "fix" a major problem by using some kind of sealant is an invitation to major issues. The initial issue has not been resolved, and others may have been created.
How did you test it "running on 3 cylinders"? Are you monitoring via a cylinder balance test? Did you run a relative compression test, or a base engine compression test?
You have major compression variances, and not sure what your leak down concern was. You may actually need an engine replacement at this point.
This forum isn't about guessing, it's about providing proper advice on how to best approach your concerns. This vehicle needs professional guidance by someone with proper tooling, skill, reference materials and know how. At some point, the "back yard mechanic" part can't deal with the serious nature of the concerns you have. This is why there are major financial investments by repair shops and technicians everywhere to have the skill and tools they need to diagnose and repair vehicles. I appreciate your concern as someone who is trying to do something without spending much money -- clearly -- but this vehicle is way past trial and error. Good luck.