I can't speak on the science of it but here's my thoughts.
GM: I've used and honestly I can't tell the difference between that and CM other than the smell
Breast Milk: I make custom batches for mamas, it makes a nice soap, but I understand the "heebie jeebies" associated with using someone else's milk. Honestly I cringe a little when testing the soap.
Yogurt: My personal favorite simply because a little goes a long way, and it's lotiony
Coconut Milk: I use this often, it's comparable (to me) to GM but without the smell
Buttermilk: My second favorite, similar to yogurt in lotion quality but it gets a bit better lather. I use this in my OMH lineup.
I don't think so, I would never dismiss a soap because it doesn't have GM - but admit I have passed on GM soaps because of the smell.
Other than subtracting from my water amount, I have never done math for using CM.
I used to agree with this, but then Dibbles did some testing with various ingredients. I discovered that a couple soaps that I thought were milk soaps were using sorbitol. So I've adjusted my thinking to "I can tell when additives are used and it's more than a lye + water + oils soap".