Well, "yeast nutrient" is basically a combination of dried yeast cells and hulls, nothing synthetic or nasty. Same can be said about most "yeast energizers" out there. I don't use DAP or nitrogen-based nutirents as those are usually produced in some sort of chemical lab. I'm not a fan of sulfates, either.
Calcium carbonate is a naturally found mineral - it's not going to hurt you and will likely increase your mead's quality - not to mention that if it is a pH issue (which I'm betting it is), a few tsps of calcium carbonate could mean the difference between a fully-fermented mead and some watered-down honey.
I would avoid raisins at this point - while they will increase the nutrients in the solution, they will also add more sugar and effect your pH further.
More yeast never hurt, really - it will all settle out in the end!
Best of luck!