How to sum all the unmerged cells next to a merged cell (repeatedly)?

Hopefully the screenshot explains what I want.

There are several manufacturers in Column B, and the products they make are in Column C.
The manufacturer's name is in a merged cell that spans all the products they make.
Column E is the # of units sold for each product.

I want to rearrange the manufacturers - currently they're in alphabetical order.
What I want is for them to be in order by highest number of units sold, to lowest.
It would be nice also to have a cell showing the totalled units for each manufacturer.

What is a logical way to do this? It's hundreds of rows so I don't want to do it manually.

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