Somebody on the MR2 site just replied with a very interesting story. He said he removed his head for the same symptoms - zero compression on #4. He also said that his HG shows the same discoloration. He claims a new HG solved his problems and the car's been fine, since. Good to hear.
That doesn't prove that my car will be fine with a new gasket. I still need to make sure the block is true, and I'm still going to replace the head because it's enough of a question mark. But I was glad to hear from somebody who had the same problems.