Good to hear you've had so much success with your Opae Ula. I wish I still had my tank setup. I had to give my tank to a friend in San Diego almost a year ago because I moved from LA to NYC. Once I move out of my Brooklyn dive loft, I'll setup a new 10 gallon tank.
As for your questions:
A. I tried this a while back. I had a neighbor with a reef tank, and he had a vial of blue-green liquid in his fridge that he said was a live culture of phytoplanktons that he would night-feed to some of his corals. I borrowed it a few times and added 2-3 drops into the tank during feeding time. Half of the feeding shrimp would swarm around the blue-green liquid as it hit the water. It looked like the shrimp liked it. Unfortunately I never followed up and found out where he got it.
B. Not once in any of my research have I ever seen plants as part of Opae Ula habitat. Java moss and mangroves do not grow in the Anchialine pools in Hawaii where opae ula live. It might look nice, but I never considered adding either to my tank. I stuck with cultured live rock only.
Lastly, I remember suggesting here that my success in getting my opae ula to breed were due to 4 things:
1. Proper water salinity and temperature. (I forgot exactly what they were. I have them written down somewhere)
2. Adequate food supply, which was twice weekly spirulina powder mixed into cold spring water and poured in, and algae growing on the glass wall of the tank.
3. Plenty of dark, interior space formed by live rock, for them to hide. In the wild, this is their natural habitat. It just doesn't seem natural to me to place them in a tank where they're out in the open with no place to hide. They need that, especially berried females.
4. Regular water changes. I know a lot of people say you don't need to, but I had great success in doing a 1/4 tank water replacement every month. I think it mimics their natural habitat, because water levels and salinity changes often in those pools, especially during rains. I think a surge of fresh brackish water with feeding mimics tropical rains bringing food to shrimp in the wild and triggers an urge to breed.
I'll post pics again once I get a new tank up and running. In the mean time, here are a few old pics of my tank back in 2007.
I would turn the air pump on for 10 minutes, 3 times a day. Contrary to what some might think, the shrimp didn't shy away from the air bubbles. Many, especially the male shrimp would jump into it and swim again the current. They seemed to enjoy it. I would run it even when there was larval shrimp present.