Good questions. At the moment, the legacy version of honeybee (the one that pretty much everyone is using on this forum) only supports up to OpenStudio 2.9.1. So, if you want the latest possible version you can have with the legacy plugin, I would recommend installing that.
With this said, I am trying to get together a new release of the legacy plugin in the next week or so and I think that I will be able to get that release to work with OpenStuido 3.0.1. In this case, you only need to worry about installing OpenStudio SDK in order to have Honeybee work. The OpenStudio application is just an additional Graphic User Interface (GUI) for editing OpenStudio models that isn’t used by Honeybee but it may still be useful for you if you want to visualize the models and default assumptions output by Honeybee.
I know that both Grasshopper and NREL’s Parametric Analysis Tool (PAT) are “parametric” but these two projects are completely separate from one another. Honeybee doesn’t need PAT and PAT doesn’t need honeybee. Frankly, if you are building your energy models in Grasshopper, you already have more parametric capabilities than what PAT is going to give you so I would say you are safe to not install it.