I can't disagree with the "check out Youtube" suggestion more strongly. It's idiot central.
I probably should've taken time to word things differently/better. Because while I didn't spell it out, I was already thinking this ... vvv
Once you've found a trainer that has the right skillset? They'll often have a Youtube channel or FB page that can refer to to supplement your work with them.
I didn't mean to imply "go to youtube and follow the first video you come across. Sorry.
However, youtube IS a great and free source of (professional) tips, especially during a pandemic when you can't just go to a trainer. And you can get good tips for at least basic tasks you can easily owner-train.
And I stand by my previous recommendation of actually spending some money. Because I do think it's crucial to get an *actual* trainer involved one way or another, directly through virtual training sessions, pay-for-training protocols, or through free content they provide.
Even when looking for organizations and trainers, one would need to do homework (as you mentioned, look for certifications etc.). There's a LOT of orgs and independent trainers especially in the US that are borderline fraudulent or just plain shit. And it's not always obvious right away - some of them are quite "famous". You'd need to do the same kind of homework when scouting through youtube etc.
Another options during the pandemic would be to look for board-and-train to get some basics done and they usually provide a protocol to continue training. However, they do tend to be real pricey and you'd also loose your emotional support during that time.
Again, *my* first recommendation would be to identify specific tasks the dog can help with. And then go from there. Some might come natural to that specific dog already and just need some reinforcement/proofing. Some might be completely new but still fairly easy (most dogs pick up on fetching medication or water etc. at a specific time or with a specific situation REAL fast). Some might be more tricky and would need professional advice/intervention/training.
Public access is the hardest to train and take the longest. If the dog is great in public, then maybe just train the tasks as you go and freshen up on the public access stuff.
I'd go out on a limb here and assume that a retired working K9 has real great obedience and PA really shouldn't be an issue. As you mentioned, might need a refresher, but that really wouldn't be my main concern with this specific situation.