Delays Log - Calculate the total number of working days each task is delayed

Hi everyone,
I have a client for which there are multiple tasks to complete. For each task, I may raise numerous requests for more information (via email usually).
I would like to calculate the total number of working days each task is delayed as a whole, as a result of waiting to receive the information I need from the client.

Please can you propose a formula which would be suitable to be in the cell I have highlighted in red in the attached example?

Please consider that:

1. The formula must not double count the working days for requests that are happening at the same time/overlapping.
2. The requests may be more than 4.
3. I'm expecting to have at least 60-70 tasks (the example in the attachment only has 5).
4. The number of requests may end up to be a few hundreds.

I really appreciate any help you can provide.
Alex.