'One or many' looks at collective agreements.
English verbs are usually rather simpler than in many other languages ~ but we still need to check we avoid using a plural verb with a singular subject, for instance. That may sound straightforward, but when the subject is a singular but Collective noun, many of us get 'interference' and put a plural verb after it.
You may be surprised how tricky it can be to get these right every time!
In part B, 'Uncle Marcus and my father WERE ..' would be better.