Missing accents and Alt / Option key functions in Monterey 12.7.6

Running Mac OS Monterey 12.7.6 on a 2015 16GB MacBookPro, I'm experiencing keyboard dysfunction in my usual user profile, both in regular start-up mode and in Safe Mode, across a variety of apps. I created a new user profile to test whether the problem related to the two Alt / Option keys (damage or dust?) or to a system settings issue: I found keyboard functions were immediately back to normal in the new user profile.

Back in my own user profile, I don't recall my system updating immediately prior to this new problem. If it is not associated with a system update, I suspect I have unwittingly pressed some key combination that has established the new dysfunction.

The symptoms are:

1) pressing a vowel and holding it to produce a list of diacritics to choose - option no longer available

2) pressing Alt / Option with a vowel to produce an accented vowel - no longer available

3) pressing Alt / Option with a cursor key to make the cursor leap back and forth from the start or end of adjacent words - no longer available

4) pressing Alt / Option with the Tab key - no longer working

I have tried various fixes suggested online, including:

1) entering the command below into Terminal and restarting

defaults write -g ApplePressAndHoldEnabled -bool true

2) adjusting Keyboard settings and restarting

3) adding a different language (French) to the list of keyboard languages and restarting.

None of these suggestions worked. Some have suggested wiping the hard drive (after backing up files to external drives) and reinstalling the OS. There must surely be an easier fix than that?

I type daily in a language using the diacritics áéíóúÁÉÍÓÚ and occasionally with other diacritics from European languages. This new dysfunction is having quite a negative impact on productivity and everyday communication with multilingual colleagues. I hope someone can shed some light on it. Thank you.