My name is Bret Grasse. I'm an aquarist at the Monterey Bay Aquarium and and manage our cephalopod collection. I've been fortunate enough to raise metasepia for about 5 years now and I can absolutely empathize with your frustrations. They are just a difficult species, no doubt about it. Please ready my D&C article for some helpful information. http://drumandcroaker.org/pdf/2014.pdf You are off to a good start by offering smaller mysid size class. It's important to make sure to increase the size of the mysid as your hatchlings grow. As a generic guideline, the prey should be about the width of the cuttlefish's head. It's also important that you don't overfeed. Make sure that the mysid are consumed before adding more. If prey remains in the hatchling tank throughout the day, the cuttlefish's predatory instinct is suppressed and they wont eat. It's almost like they get overwhelmed with too much prey. Hatchling tank walls should be non-transparent for improved prey recognition. Similarly, prey items should be fed prior to feeding them to the cuttlefish. This will make them non-transparent for easier detection and make them more nutritious. It's normal for baby meta to not eat for the first day or two after hatching, so try not to overfeed in these first few days. Premature hatching is a big problem, you need to start off on the right foot with a healthy fully developed hatchling if possible. I've used batadine baths and a solution called Revive that I would highly recommend for the eggs. Revive is made for corals and it irritates copepods, flatworms etc. When you use it on cephalopod eggs, it does the same thing. Without all that biofouling, the egg's external tunic is preserved for longer and allows for full development before hatching. I perform these baths every 4-5 days. Correct artificial incubation is also important to prevent premature hatching. So adjusting your incubators also may help.
Hope this information is helpful. Good luck!