I have 39 returned WU's dating back to 2015 that have not yet been validated. When are these likely, if ever, to be validated?

Never.
The word "Validation" is a BOINC process where 2 results from 2 different computers are compared to see if they're the same. (And therefore correct.)

ALL climate models using the same starting data, have been shown to produce slightly different results on different computers, so this process isn't used in this project.
It's what we have trickle_up files for; to show that work has been done by a given computer, and therefore deserves credit.
Which is calculated once a week, starting early on Saturday, UK time.

(And therefore correct.)

Just to add to what Les has said,
All work which gets far enough to produce trickle up data gives credit.

To see which data is useful from a particular batch a complex statistical analysis is used and as I understand it anything too far out may be discarded.

Occasionally a task will go even further outside the given parameters and produce an impossible climate such as a negative atmospheric pressure in which case you get an invalid theta error when the task crashes.