From your post, my first thought was that there's something up with the old man. Even if your Bella had a go at him straight away out of the blue, why would he snarl and swear back at her? That's not really a normal response.

My dog has a thing about older people. I think it's because of the way 71yo FIL tried to 'play' with him when he was a puppy, and because the old guy who comes to read our electricity metre is nasty to him. So now I'm extra careful with him around older people, but don't want him to get snarky with kids and others either.

At the cafe, how long is her lead? If she can't really get out far from under your chair it shouldn't cause much of a problem. And when out walking her, if you see someone coming up to pat her I'd just ask them not to, or to be careful. I do that with our dog - he's a big lab, and when little kids come running they don't realise how excitable he can get and I don't want him to hurt them. I usually put myself between him and the person and then just ask them to be careful. Obviously if he showed signs of being unhappy with that person I'd leave with him.

Other than that though, the vet probably will have some good suggestions, and I like the idea of seeing a dog behaviouralist.