OK, so this is way simpler than your attempted solution.
You already have a field on the Document for [Related Project] which may or may not have data.
Do a lookup from Tasks down to Documents for [Related Project], but call it [Record ID# of Project From Task]
Make a new formula field called [Record ID# of Project]
The formula will be
IF(
[Related Project] >0, [Related Project],[Record ID# of Project From Task])
Then make a new report link field on Projects (or re-configure the one you have already) and on the left side configure to [Record ID#] and on the right side navigate back to you same app and locate the field [Record ID# of Project] on documents,
Put the Report link field on the Projects form and and y'er done!
