Good to hear! I agree, it’s not a great situation but we can at least make the best of what we got. 
Yes, you can update it from the Tag Panel but it’s a far easier workflow to store all your actual metadata into DIRECTOR tag etc. then update ITUNMOVI via the action. If you start manually editing ITUNMOVI with different data from your tags, it’ll get confusing and you’ll risk easy mistakes in the XML’s formatting.
tldr; No, try \\ ; or / using the action, but the action could support , if necessary.
iTunMOVI as an XML file and as an ITUNMOVI tag represents separate “directors” values as blocks like:
<dict>
<key>name</key>
<string>CamdenTommy</string>
<key>actorof</key>
<string>Batman</string>
</dict>
The DIRECTOR tag - or rather any tags in Mp3Tag - have separate values represented by \\, e.g.
CamdenTommy\\arb
It’s the correct way of doing so and you won’t see \\ in other software, like Windows File Explorer will recognise these separate values but represent this by ;
… but surprise surprise! Not all software supports it, so the other options are ; or / which aren’t really separated. They’re just visual representations and still have their own problems.
The action currently will look for \\ ; or / in your tags to separate values. It’ll then convert each value into the correct format for iTunMOVI. I'd strongly recommend changing comma-delimited tags to any of these as there’s more potential for names to include , although that is mostly musicians like “Tyler, the Creator” or “Earth, Wind and Fire”.
No, sadly not by ITUNMOVI or any local metadata. 
I’d say its still worthwhile storing it in a custom COUNTRY tag. Hint hint: you can then copy it into COMMENT and it’ll get picked up by your OS’s indexing system so you can search for your media by country that way 
As in multiple files? It’ll create a separate ITUNMOVI tag for all those files so yes, performs it individually. 
Not sure myself, though I was removing/re-adding libraries while testing to brute force changes. 
Strange. Is this in Mp3Tag or Plex?
ARTIST is an official tag that lots of software likes to use in different ways, like musicians, actors, author, director etc. That’s why it’s better relying on distinct tags especially when it gets put into a definitive “director” value using iTunMOVI XML.
ACTOR isn’t an official Mp3Tag tag nor an MP4 tag but it is an official MKV tag and can be used properly just by calling it that label in Mp3Tag. Again, the action uses this instead of ARTIST for the same reason of distinction as DIRECTOR.
CAST is a totally made-up tag that was just an example. You can label tags whatever you want for you own workflow but it’s better to try and use as many official ones where possible (it’s not always possible).
Just a video file purchased/downloaded from iTunes, either MP4 or M4V format. Will normally come with metadata including ITUNMOVI that we can use as a reference to how Apple does their metadata.
Just type PRODUCER as a custom labelled tag, set it up in Tag Panel, use the action and you’re sorted. 
@ohrenkino‘s right to point out there’s other values that can be seen as a “producer” like COMPOSER, LYRICIST and PUBLISHER depending on software/your personal workflow. For Plex, it must be read from ITUNMOVI.
It is a bit weird how it asks for “screen-writers” for a writer (or writers) but that’s just the way it is.
Thank you!
