How to SUM all cells in a column that are equal zero in the same row but previous column? (Google Sheets)

This is my first question on StackOverflow.

I am trying to SUM all cells in a column that are equal to zero in the previous column in Google Sheets.

I can easily SUM each column or the differences between them. However, I also need to SUM non-zero cells that are zero in the previous column.

For example: I need to SUM cells C4 and C16 as B4 and B16 are 0, but C4 and C16 are not. I need to come up with a formula that will do this for every column starting with Column B.

The result of this formula for Column B = 0 and Column C = 1300 that I would be able to monitor.

Ideally, I also need to SUM cells that are greater (or less) than the cell in the same row but the previous column.

The result for Column D = 250 (D4-C4+D16-C16) but Column E = -250.

I have been struggling with this for days now – I've tried Indexing, matching, but never cracked any of these problems.

Here's an editable Google Sheet. https://docs.google.com/spreadsheets/d/1rA1VMfjsIajVrwoYN7p_uovzze9GZQbsYnqz200LkrI/edit?usp=sharing