Once I understood the data well, I realized that the raw data had to be flattened out to expand collections (like KB) at the row level into their own row so that everything has a single value in each row. Then, the grouping is easy.
It made more sense to allow grouping not just by KB but by other columns like Product or CVE. The Group-Object works fine for most cases but since there will be duplicates after the data is grouped, it makes it easier to just do it with HashTables.
Jana provides the entire solution on his site. When reading it, I felt the urge to switch to a language which offers easier pivoting and aggregation, but the code was clear and understandable.