Can I store a Citation resource on a page different from where the actual reference is made?

Yes. A Citation resource can be stored on any page and is accessible from any page through the Citation key declared by the resource.

Can I add the same reference multiple times to a (or different) text source?

Yes. Using the same [[CiteRef:: ...]] reference at the position that needs citing is all that is required.

Is it possible to use a resource defined by <ref> (Cite) with #scite?

No. Due to a different technical approach resources declared by <ref> can not be used by (or in) Semantic Cite.

FAQ

How to handle different authors

There are various ways of making different authors available to the semantic search and as ordered output.

For example using a parameter authors that is matched to a property Has author (to contain all authors in clear form) while an author parameter (not matched to any property) is used as identifier so that the template formatter can generate the expected ordered output from {{{author|}}} without having to apply a complex parsing process.

{{#scite:Watson and Crick, 1953
 ...
 |author=James D. Watson;Francis HC Crick|+sep=;
 |authors=Watson, James D., and Francis HC Crick
 ...
}}