There were more edge cases and wrinkles in it than I remembered, but the new processor version is now ready for testing.

See how it works on the full version of your document. If there are glitches, let me know. (Can't guarantee immediate success, but the "sep is undefined" failure will definitely go away, and the hack that gave rise to it is no longer in there.)

You install the Propachi Vanilla plugin in Zotero Standalone. Tools->Add-ins->Gear->Install Add-in from file.

@fbennett "sep is undefined" problem is gone, immediately. Thank you very much for the efforts and time you put into solving this problem!!!

I want to signal about a minor issue. As a reference style I have selected the Chicago manual full footnotes in the settings. Now some of the references appear partially (such as, the author's name and the title, the title and the publication date or the title and the pages—it's unsystematic), among them regular publications in English. The issue is present in the old document with which the "sep is undefined" problem started and it repeats in the new document I just started. Do you think they are related?

@hayarp.papikyan: Very glad to hear that it clicked! The short-form references almost certainly follow the rules of the style, as @bwiernik says. The changes in the new processor version affect only the formatting of text inside titles, not the formatting of citations.