I also have this problem, many completed WUs ready for upload for 1+ week.

Do WUs completed before deadline, but not uploaded before it, still contribute, or are they cancelled / wasted...

(edit) upload is delayed only, it seems, on my W10 box. Other machines seem to be uploading results normally

I've noticed quite a lot of strange hosts without credit but with around 1000 tasks in progress

There are two likely explanations for this:
1. Folks participating in the Pentathlon who have never contributed to Universe before. They may be holding the tasks in reserve to release them at a certain point (three days out of the two week event have bonus points for results returned).
2. Some individuals use underhanded tactics: they create BOINC clients, load up the client with as many tasks as they can, and then simply hold the tasks until timeout. They have no intention of ever returning the tasks; they are trying to preclude validation of the work units for those tasks during the competition to reduce the overall credits earned by participants, thus increasing the relative value of credits within the competition timeframe. Personally, I think if that behavior is caught and confirmed that they should be banned. Clearly, the number of credits means more to them than the science, so hit them where it (presumably) hurts them. Turn their BOINC account and all its credits to anonymous.