Technically speaking, à is a grave accent. á is acute.
That Unicode hex input must be something invented for Windows users. That used to be (still is?) the only way that Windows users could enter high ASCII characters. Clearly, Apple seems to have broken this input method in Monterey. I was able to get it to work for other characters around that one, but not with that specific character. But curiously, I couldn't get any of it to work at all at first. It took some effort. And by the time I did get it to work, my emoji and symbols viewer was completely broken. It won't display at all anymore. I guess I'll have to restart to get it back. Sadly, this does not surprise me at all. Monterey is one of the buggiest Apple operating systems that I can remember. This is just one more oddball for the list.
My recommendation would be to avoid the Hex input method. As I said above, that is clearly something invented for Windows users. The traditional method for Mac users is as follows:
option c: cedilla ç
option `: grave accents à, è, ...
option e: acute accents á, é, ...
option u: umlaut/diaresis ü, ï, ...
option i: circumflex î, ô, ...
option n: tilda ñ
option q: œ
option ': æ
option s: ß
option/shift option \: «»
This is the old-school Mac way. It heavily favours European languages, but that should be fine for you. It may take a bit of time to develop the muscle memory, but once you do, it never goes away.
Later, Apple added the press and hold method, but people report that this people stops working half the time. This traditional method has never failed.
Your custom keyboards are a different problem. They shouldn't be dependent on this hex input method. They should use something a little lower level and generate the specific unicode characters using the correct USB API. This would be the "correct" solution. An easier solution for someone that only needed European languages would be to just change the mapping to use the traditional Mac mapping. Then you wouldn't need the Hex Input method at all.
