Try testing if people can comprehend the diagrams by asking a question after showing them a somewhat complicated one. You'll likely be disappointed. We don't seem to be using them because they work.
> As it happens, most people understand easier a standard notation that can refer to, and get a book at the library, than NIH boxes and lines.
I do not believe this for a second. You're possible arguing against a straw man.
The alternative to UML isn't a "NIH UML" which uses a different array of arrows. It's labelled boxes and lines. You don't need to go to a library to understand the diagram because the diagram is self-explanatory.
I don't think the standard matters, so much as the visual diagraming not seeming to work well at scale.
A drawing showing the relationships between database tables seems to be comprehendible to people, until you get to over 7 tables or so, and then they don't appear to be able to make sense of it. Real databases often have magnitudes more tables than that.
You apparently have something you can do a trial with. Show it to people, give them time to digest, and politely quiz them a bit to see if it worked.
Just a couple of months ago I finalized yet another archictecture document, with enough class diagrams, sequence diagrams and use cases on it.