[mindnever]

Is there something I can do for the missing cast thumbnails ?

No. Unfortunately Kodi is not always able to download remote artwork. The artwork may have been removed (in which case you need a new remote url), or the remote web site hosting the artwork may not allow mass scraping, so keep trying and it might work a bit later.

Rate limiting, ideally in Kodi as it's Kodi that performs the actual download from the remote site, or in the texturecache.py script might help - you could try downloading cast artwork using only a single download thread in texturecache.py with @Download.threads=1 (edit: that's download with a lowercase d - thank forum) and see if that helps. Or try @singlethread.urls=ia.media-imdb.com which limits the sequential download to the specified (comma-delimited) domains.

Not sure if Universal Movie Scraper actually downloads the artwork - it may just associate the movie with the actor url, then the artwork is downloaded the first time you view that actor thumbnail in the GUI (at a time when the remote site is allowing the reduced number of requests from Kodi). 

I will try the download.threads=1 option.
Although I did try to cache/download cast for a single movie (6-7 actors missing), without success...

Thanks again Milhouse, I really appreciate your work...