If the eggshells are crushed up they will fizz more violently and more quickly (which is what happened with you) but it ought to make the process be completed faster. If the eggshells are put in whole or in big chunks the process will be slower, not fizz quite so violently but it will take longer for the process to be completed. I'd suggest that you decant the whole thing out into a bigger, wider open-mouthed jar or container with more air space at the top. That will reduce the surface-tension of the surface at the top of the liquid and the liquid will circulate round and round, up and down inside the jar while the shells are dissolving, instead of being forced upwards and out through a narrow neck. The oxygen being generated has to have somewhere to go so I think you need to leave the cap open a bit to allow the oxygen to escape.
I know it's not exactly the same thing but let me use a little simile here. If you have a fish tank that is tall and narrow with not much air space at the top, and another fish tank that is shallower and wider with more air space at the top - and they both have the same amount of water and air-bubbles being generated by a pump - the water in the short, wide tank will circulate more efficiently and less violently than the tall tank because there is less surface-tension at the top to restrict the circulation of bubbles.
I don't have Susun's book and am wondering if you (or anyone else) could give me the recipe for old sourpuss mineral mix, I'd like to try this myself.
Good luck with that.