This code is too big for me to digest it, but the basic problem is that
```
(let* ((default-name (ly:grob-property grob 'text))
(new-name (assoc-get default-name newnames)))
```
is doing a lookup in your `newnames` alist with the value of `text` as the key.
In more recent versions, `text` is not a string but a full markup, which you
can see if you add `(display default-name)`. So the lookup fails, `assoc-get`
returns false (`#f`), and you set the grob's text to false, which leads to a
warning because false isn't a markup.
The quick fix here is to change
```
(let* ((default-name (ly:grob-property grob 'text)))
```
to
```
(let* ((default-name (markup->string (ly:grob-property grob 'text))))
```
in order to first get an approximate string representation of that markup that
corresponds to the keys in your alist.
But instead of this fragile method of changing note names, it would be a lot
better to write a custom `noteNameFunction`, e.g.,
```
\version "2.24.2"
\new NoteNames {
\set noteNameFunction =
#(lambda (pitch context)
;; Write logic to compute the note name from `pitch` as a markup.
"TODO")
c' d' e'
}
```
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