Pluralization: backward and backwards in context

Unless I'm using "backward" and "backwards" wrong, I can pluralize it when the subject is not plural and vice versa.

For example, correct me if I'm wrong:

One movie.

If you watch your favourite movie backwards, what is the plot?

More than one movie.

If you watch your favourite movies backward, what are the plots?

I've never stopped to think about this before and am looking for some quick enlightenment as to its validity.

I've seen this question based on the same words but believe it's a slightly different context.

The -s in backwards is not a plural suffix, but rather an adverbial suffix, originating in the adverbial use of the genitive case (the genitive case is the origin of modern possessive 's). So there is no relation between backwards and plural nouns.

There's really no difference, except that certain idioms (like bend over backwards) call for one or the other. Dictionary.com agrees. In some areas/dialects/social groups one will be used over the other, but they're basically synonyms at this point.