Turned my comment into an answer so it can be downvoted as well.
I'd vote for option 3, that is to say a new tag els-cas. It seems to me that elsarticle
and cas-...
classes are sufficiently different to warrant a new tag. elsarticle
is thoroughly LaTeX2e, while els-cas
makes heavy use of LaTeX3's expl3
and xparse
.
It might help to add cas-sc and cas-dc as tag synonyms for els-cas so that people using the document class name as tag are automatically pointed into the right direction.
My justification for that goes pretty much along the lines of your reasoning in the last paragraph. Especially if Elsevier intends elsarticle
and els-cas
to be used in parallel for some time it is useful to have questions properly separated between the two classes.