For what it's worth, i don't think it's necessary to create a new field, or use a complex expression to get this done. Here's what I'm doing with this...
In Theater View options, I created a sub view called Series. For the fields in that series, I use an expression (not a field), which is If(IsEmpty([Series]), [Name], [Series]) and I called it Series, so that is what is displayed in Theater View on the roller. it could be called anything. I use the Series field for the name of any Movie series (Star Wars, James Bond, the Avengers, etc)
then under "Set rules for file display..." I have...
Episode (a-z)
Date (a-z)
Name (a-z)
So if there is an episode it will sort by this first (if you want to force the Episode 1 of Star Wars to show first, instead of Episode 4, which was actually released first), otherwise it will sort by date, otherwise by name.
Works perfectly, and I only use the one expression above. I actually don't even need that expression, since I have a Series for every movie already populated, I could just select Series, instead of the expression, but I will probably stop filling in the Series where unnecessary now, since the expression works perfectly well.
Anyway, just another way to skin a cat