I see in the release notes that there is a way to merge models, but I'm not sure how to do this.

Unfortunately, I have an 1800 word vocab deck and it's split across 11 models, meaning I can't easily reformat for something like the Japanese language support plugin.

Anyone been able to delete models WITHOUT deleting the cards, or merging models with the same number of fields?

You should really ask this in the Anki Mailing List (you're more likely to get help that way).