As an update... oldest zoe lived to be about 2 1/2 weeks old before, presumably, dying. Been trying to do some more research. I've got some theories...
Amano shrimp may have a wide range of when they give birth due to the raining/flooding season. I've seen people comment on how their berried females hold their eggs for a really long time, but some people have noticed that once doing a water change, their females release their eggs and they have baby shrimp! Dropped eggs, even without any current, can still hatch. So my thoughts... Flooding season results in the release of babies (and possibly eggs) which will then very quickly travel down to the ocean. About a month later(?), when it's the "dry season", the newly morphed shrimp can then travel back upstream.
It can take anywhere from 15-50 days for amano eggs to hatch.
If my understanding is correct, zoes do not require food while they are in freshwater as they are eating the remains of their "egg yolk". They may not be able to eat, anyway, until after a certain stage in growth, which can only be achieved in saltwater. That is to say that they can't molt and evolve if they are in freshwater.
Japanese have found that highest survival rates for zoes is at 16.9 ppt, where-as many hobbyists notice highest survival rate at or around 35 ppt. In the same study by the Japanese (which I have not read), shrimp metamorphosis faster in higher saltiness. I'm not sure if the difference here is due to nutrient content of the water? As in, the higher nutrient content of the water, the less importance salinity may be?
The water the zoes are in, in the wild, may have .2 to .6 phosphates, 2 to 6 nitrates, although I could be completely off here, too. I'm just thinking in terms of using tank water plus ocean/marine salt vs RO water and salt. My first set of zoes were in RO + Salt with a 30-32 ppt salinity, and the last one dyed around 2 1/2 weeks of age. The second batch are being put in 1/2 tank water and 1/2 RO + extra salty water.
Zoes should probably ideally be kept in temperatures between 68° and 77° F, but no less than 64° F.
Still having issues with zoes surviving, but the newest zoes should be no more than 1-9 days old and I'm still waiting on a hatch from the remaining female. I had 2 berried females, only 1 is now berried, and at most, I only got around 20 zoes...
I think I need more amanos.... and more tanks.