I would make the brain last priority because fake cheap brains are so easy to come by in Halloween stores. If I had to make one, I doubt that I would go to all the casting trouble. Try carving one from light foam rubber, or shape it using one of those heated pencils designed for cutting foam.

If you do this as you say with expandable foam (like the kind made for insulation) your brain will be hard like stirofoam instead of squishy.

I recently discovered that the liquid rubber sold in home stores (for dipping the handles of tools) makes a convenient, thick brush-on coating to seal a rough prop in smooth rubber. So you might form the brain from something lightweight like papier-mache with a foam center, or better yet something flexible like bumps of clay or papier-mache attached to a bean bag to resemble the brain shape, and then brush the rubber coating all over it to both toughen it and make it waterproof. To me that still sounds easier than creating molds.

I would be more fussy about the head, creating appliances for both the human and the dummy from the same castings. Make sure anything hard to replicate is made "fake" on the real monster too. For example, the actor should have crappy wig hair instead of real hair visible so that you can match it exactly on the dummy.

Lastly, consider an optical illusion in place of creating a dummy at all. Suppose the doctor encases the head in some kind of clamp thing to saw off the top of it. You could use a mirror trick to show the actor's head cut off at the point of the clamp division, so that only the dome of the top of the head has to be in fake form above that division. It's easier to design and build a slab or dentist-chair type thing to control the setup of the mirror and the actor than it is to design and build a realistic human head that has to match a living actor.