Hate to say it, but...
Do what your insurance carrier says. Sadly, if they have a term in their insurance policy you don't abide by, you don't get covered. i wouldn't ask them specifically about OC or firearms, just get a full and complete copy of the policy.
"Under the influence" has different meanings for different folks under Idaho law. For a minor, it's .02 for a DUI. For someone with a CDL, it's .04. For the rest of us, it's .08. It gets tricky because the rules of statutory construction require that statutes be read "in pari materia," meaning on the same level, or such that they can be construed harmoniously. Since there is no accompanying statutory language in the CC "under the influence" statute, we have to look elsewhere for guidance, and the only place it appears is in the DUI code, or the general alcohol provisions, which are of no help because they only use the wholly indefinable term "intoxicated," for which there is no objective standard. Accordingly, the DUI laws are all we have that has an objective standard, and you should look to those for guidance.
Don't have to worry about the .02 level for minors, unless you plan on serving them.
It gets trickier when you deal with those with a CDL versus a standard drivers license, but it's reasonable to assume that the statutes regarding both, particularly the CDL's, place the burden of responsibility more so on the individual driver to determine whether they meet the DUI statute's level of intoxication. Logically, it would apply to those who are OC-ing, rather than the server as well.
Note that your servers, having presumably been through the TIPS program, know to not over-serve and to cut off those who get a load on, so to speak.
The only ones who have standing to go after you would be ABC/state/county/city for over-serving, or anyone injured by the OC'er as a result of being served to intoxication, which we know is highly unlikely given their law-abiding nature.
I'd say serve the OC'er the sandwich and a beer or two - they likely aren't going to be trouble, particularly if they're eating high-protein food while quaffing a coldie or two, but make sure when they ask they look/sound/smell sober. If they were drinking before they arrived, you don't want to be the icing on the cake or the over-server. If you don't feel comfortable serving them anything, it's your prerogative as a private property owner/business owner to deny them service. If they get into a huff, and if they've obviously already had a few, you don't want them as customers - send them away. Doubtful you'd ever have to with conscientious OC'ers like those on this board.
Just be smart and safe - as a commercial entity, you have a responsibility to the public that is greater than John Q. Public regarding alcohol sales, and are in the best position at the time to prevent injury (DUI, etc.), so are expected to act responsibly.
I'm sure you'll be fine - you have the smarts to ask around beforehand, which means you are a conscientious and community-minded businessperson, and are to be commended.