How can a make very thin layers of silicone?

If you are spraying a plastic membrane like Super Baldiez or something like a vinyl cap plastic on silicone you would only want to release the silicone if you want that membrane to appear free-floating or be easily separated in the final effect. Without a release they will lightly bond with the silicone, but should still peal up easily.  You'll want multiple coats (possibly 5+) to get a thickness of cap plastic that will not easily tear.  But that all depends on the effect you are going for.  You can try different materials, and different numbers of coats to see how the materials react in your particular application.

For a release agent you could either lightly brush on vaseline or use a release spray.

Also keep in mind that you would need to repeat this process to replace the membrane every time it is cut or damaged during use.  So you may want to have a duplicate of each surface that will have a membrane and use that surface to spray silicone membranes on that you could then carefully remove and stretch over the working model.  That way you can have plenty of replacements.

If you are spraying a silicone membrane onto a silicone surface you will need a thorough release, as silicone sticks to silicone

For the wet look there are plenty of options.  Glycerine is a common one.  The most common is probably KY (or other water-based lubricants), as they give you a good wet/slimly look without being too thick, and can easily be thinned/re-activated/washed off with just water.

If you want something more thick and stringy like mucus, you can go with "Ultra Slime" (a brand of slime you can get off the shelf) or if you want to make it yourself you can use Poly Lube or J-Lube.  They are a polyethylene polymer lubricant often used in livestock birthing.  They come as a dry powder that you add to water.   One great thing about going that route is you can tailor the thickness of the slime to your application by adding more or less powder.

I would go the water based route over oil, as oils are more difficult to clean up, and can get rather messy when transferred to other surfaces.

Start with the Karo in a large container, and add water until you reach the desired thickness for your application.  Then add the red food coloring until the color looks right.  Test the color density by spreading a small bit out on something like a paper plate.  The blood will look much lighter when in a thin pool/smear than in your main container.  Then add green a drop at a time until the desired shade is reached.