This is a great post as your challenges are very common. I have discovered the mini Miracle Nipples and the regular Miracle Nipples to be a Godsend. The tinies don't get mouth irritation like they do with standard nipples; the MNs are pre-holed and seem to cause much less gas; the nipples are sturdier than average and work both with syringes and bottles. I know that the milk temperature should be at 100°F; to keep it warm during burping, pottying, etc., you can wrap a heated rice-or-cornmeal sock around the bottle or syringe; a bottle can also be immersed in a cup of hot water.
Sometimes kittens need extra time on straight formula. I think that their insides might not yet be fully developed; because cats ovulate after mating, the kittens can be at different stages at time of birth.
#FosteringaPet