1: Not sure why it would be bitter. My guess is that fermentation is still so young that you are tasting byproducts of that. The yeast haven’t had nearly enough time to clean up after themselves.
What hops did you use for bittering and at what weight/time?
2: Putting in another batch of yeast now is kinda tricky. The problem you will run into is that there is no oxygen, little nutrients left, and there is a presence of alcohol now. I would start by increasing the temp and rousing the yeast by giving the carboy a swirl… if you fell that maybe your fermentation has come to a stand still and the yeast have given up.
Some have results with champagne yeast because they are ready to pitch into an environment which has very little nutrients and the presence of alcohol. Champagne yeast and wine yeast can tolerate these conditions.
3: The flavor profile of the cocoa nibs may round off the edges, but I wouldn’t think they will completely get rid of the bitterness. How long Have you been in primary for? I wouldn’t transfer anything into secondary until 2 weeks of primary. You can put in your cocoa nibs then if you’d like. Some wait even longer. There is no hard fast rule on when to add them. I just wouldn’t add them to primary when it’s still bubbling so you don’t lose any aroma during the release of CO2.