Theoretically, the MM100 will give you slightly more open texture due to gas production in diacetyl, which is responsible for the buttery/creamy aspect of the flavor profile. The gas creates little bubbles which are the holes or eyes in the cheese. We are not talking Emmental kind of big holed open - just random tiny eyes here and there. The formation of these eyes depends on having an elastic paste texture to form permanent hole openings. In the case of Saint Marcellin the cheese is dense and the geo breaks it down into a nice thick goo rather fast. Being so thin, the whole cheese turns into this soft substance. The bubbles collapse as the paste can't sustain them open. I personally think MM100 gives it a fuller, broader flavor and improves the texture and aging too.
Your milk sounds wonderful!!! I am jealous! If it's good and tested, then you shouldn't pasteurize it any more than you pasteurize and organic head of lettuce! Anyway, your pasteurization sounds like a very gentle batch process. With such high milkfat and no homogenization you can probably eliminate the Calcium Chloride from your make altogether. If you add too much you will end up with overly stiff curd and minerally cheese. Calcium has this soapy bitterness about it if too if too much is present. I say, let the milk do its work without the added calcium. If you get too much - add more moulds or better yet, use the excess curd to experiment with something wild that you would not otherwise dare trying, perhaps with one of the other moulds you have laying around... In short, don't decrease your milk. Eliminate the calcium. The smaller yield you get will be of much better quality.
Rule of thumb: If you have a clean break from a curd where no calcium was used and your whey is clear/greenish (not milky) than your curd quality is very good. No calcium needed.
By the way, if you mold a Saint Marcellin to the top with proper curd, it should end up as the thin disc it was meant to be. If you over-load the mould (for example waited for it to go down a bit and then topped it off with remaining curd) or your curd is too stiff (too much fat or too much calcium) than you will get these thick pucks that you didn't want. If that happens and the cheese is still in its early draining phase, you can cut a 1/3 of each of 3 cheeses to make one 4th cheese of equal height. As long as this is early enough and still very wet, these 3 slices will fuse together to become 1 cheese. You will hardly be able to tell which of the 4 this is after the rind grows!
Here is a St. Marcellin I did this past summer, from a past post. This one was done with MM100 by the way. As you can see - no open texturehttp://cheeseforum.org/forum/index.php/topic,8555.msg60881.html#msg60881