The ex-boyfriend 'lawyer' does not 'represent' him. If he did, his name/office would have been on the letter you were sent.
If I had a friend who was a professional circus clown, and they opened my door to a police officer (or anyone else for that matter) in full makeup and pulled a hundred handkerchiefs out of their pocket while saying "Hello, this is my client" and honking their nose, their 'representation' would carry the same amount of legal weight as that of your neighbor's 'lawyer' - none.
Opening a door is not a legal matter. Speaking while not under caution is not a legal matter.
You also seem caught up in the notion of having some kind of criminal record that you had to clear. As PP have pointed out, you seem to have misunderstood what an NCHI is.
Apply common sense here: If the mere recording of an NCHI would cause someone to fail any kind of vetting, everyone with a grudge would be filing them and tanking people's careers.
All that aside, you accuse your neighbor of gossip, while stating that the reason you know his ex is a 'high-flying' lawyer is via gossip from other neighbors and conversations heard over your garden fence.
One minute they're flying drones over your house, the next you're in theirs watching the 'lawyer' open the door to strangers and introduce your neighbor as their client.
I think it's likely that there's a serious case of Soap Opera-itis going around your little group, and both sides have got so deep into the gossip/drama that you can't back down.