I could almost guarantee your wrasse was the culprit. Six Line wrasses are notorious for picking on fish that are added to the tank after them. This is why it is suggested to make a six line wrasse the last addition to a tank. I've had them kill fish on me as well. You'd be surprised what those tiny fish can take down. It could have been your gramma, but my money would be on the wrasse. Was the wrasse the newest addition before you tried the cardinal?
With Bangaii Cardinals, captive breed specimens are far easier to keep than wild caught. Many people think this has to do with collection methods.
They can thrive singly in a tank, but many note that they do better in pairs. Do not keep an odd number of these fish. Unlike PJ cardinals that will form small groups, Bangaiis form pairs and will beat on a weaker fish that is not paired, and often kill it. Try not to buy a Bangaii (or any fish) without seeing it eat first, and with Bangaiis, really try to find a captive breed or tank raised specimen.
In your situation, you really are probably going to need a pair. The wrasse should be less aggressive towards a pair than he would be with just the one.
It would be a good idea to set up a quarantine tank. Besides the obvious disease and parasite reasons, a QT allows you to make sure the fish eats. You could also use it as a "time out" for the wrasse. Take the wrasse put, add new fish, allow them time to acclimate and find a territory, and then add back the wrasse. The wrasse will have to readjust to the tank and find a new territory again and thus usually makes them less aggressive. Six lines are terribly hard go catch though.
As for the shrimp "messing" with him, he was probably trying to clean his wounds. He may also have had a parasite (ich) that the shrimp was trying to eat. If he never ate, he may very well have had a parasite. I highly doubt the shrimp killed him.
