You can get the content item ID from the Pram QueyString and then call GetMetadata("Category") of the IContent object which will return the name of the category. In contentItem preview there is a method ShowMetaData that you could access and get the contentItem and its metadata (Category) directly.
Another option is adding the category name as a QueryString parameter to the url and get the categoryfrom there.
To navigate from ContentItemPreview to CategoriesView you have to use CreateRootViewCommand
b.NavigateUrl = CreateRootViewCommand(
You can gather more information at Sitefinity 3.6 new backend architecture - How to navigate among views: What are commands?
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