There was a popular business management book twenty three years ago - Who Moved My Cheese? that might provide some guidance on adapting to circumstances when people don’t see things your way.
I found an OSHA document way back from 2005 on OSHA policy on dealing with hearing impairment in noisy work places that predates the letter link that I sent you so that might have some helpful information in it:
The link to the corresponding web page that used to deal with the same topic is broken and no longer works: https://www.osha.gov/dts/shib/shib122705.html so I don’t know if the document that I’m referring you to is just an archival copy and no longer official OSHA policy.
There is also this slide talk from a consulting firm(?) as to what can be done for hearing impaired workers in a hazardous noisy workplace, which references the 2005 OSHA document: Effective Workplace Accommodations for Employees with Hearing Impairments pt 1 of 2 051619-Goddard.pdf (unt.edu)
I guess one thing that you could do is write or otherwise contact the various leading HA OEM’s: Phonak, Oticon, Sivantos(Signia, Widex, etc.), ReSound, Starkey and ask them if any HA, particularly custom-made ITE from a cast-mold would have an effective NRR/SNR rating or once custom-made could be rated. If you can get answers from at least a couple OEM’s, that would give you an idea of which way the wind usually blows. These companies or their representatives must have to deal with quite few folks each year in situations similar to yours.
It won’t do you any good to find a solution but not have a job so in the interest of keeping your job, it might be a good idea to go along as much as possible with what’s requested until you can muster a very, very convincing counter-argument. The best would be an alternative that OSHA has officially approved of for you. So I wonder if there is currently any office at OSHA that is so far removed from local interaction that your employer or local OSHA representatives don’t know about the department but the folks there have recent experience with providing solutions to situations like yours that you could be happy with - the previous OSHA letter that I referred you to was from The Directorate of Enforcement Programs - that’s pretty high up - but maybe that’s some place to start at OSHA? Good Luck. Hope you can find a win-win situation as it’s easiest to sell that sort of thing.