I think my answer in the previous post might have been slightly incorrect in English and therefore confusing you. I personally don’t make a real difference between “Nobody arrived” and “Nobody has arrived”, and I think that’s were I’m wrong. Actually, the real meaning of “Personne n’est arrivé” depends of the context.

You could be telling a story that happened in the past :

J’attendais mes amis mais personne n’est arrivé.

-> The action was in the past and is finished, nobody did ever arrive. We use “imparafait” because it’s an action that spans over time. I would translate this to :

I was waiting for my friends but nobody (ever) arrived

but I guess it could be translated to

I waited for my friends but nobody (ever) arrived

I couldn’t tell if both translations have the exact same meaning though, I would maybe mistakenly use either one or the other without further thinking.

You could also be telling about something happening now :

J’attends mes amis mais personne n’est (encore) arrivé.

-> The action is in the present, I’m still waiting, and nobody arrived hitherto (and if anyone had arrived, that would have been in the past from now, so we use “passé composé”). I would translate this to :

I’m waiting for my friends but nobody arrived (yet)

I’m no translator nor French or English expert, but I think tenses in French and English just don’t always have a direct translation and that may lead to some confusion. French doesn’t have “continuous” tenses for example. Taking my example from previous question remark :

Je bois de la vodka

could be translated to “I’m drinking vodka” (=> right now) or “I drink Vodka” (=> usually; I’ve nothing against vodka as a general matter)

As of the difference between various past tenses in French, I think those 2 schemas combined summarize it well :

Imagine that you just arrived to an appointment with some businessmen. You are expecting five persons to join the meeting but no one is there yet. A friend calls you to check on the meeting and asks you if the others arrived yet by saying Les autres sont - ils arrivés?. You can answer personne n'est arrivéwhich would be perfectly correct. But you would still be expecting them to show up anytime. So your statement:

In French it would be Personne n’est arrivé which to me is something happened once and complete in the past

Is incomplete: it can be something that happened in the past but is still ongoing.

Now let’s assume that two years later you are relating this event. You could say Personne n'arrivait, et je m'impatientais. It means that action happened once in the past and it is not continuing in the present, but still it is part of a sequence of actions that all occured in the past once (it could be followed for example by ...tout frustré je décidais de m'en alleror a happier situation: ...enfin ils arrivèrent tous en trombe et s'excusèrent du retard)

I don’t know exactly why this is.
But

Personne n’est arrivé

is an observation at point T in time. Nobody is there.

Personne n’arrivait

It gives the idea of continuously observing the situation. Nobody was coming. There is the feeling of waiting for it to happen. The imparfait has this notion of continuity.
Well, at least in my mind and for this situation.

By the way, I’m a native french speaker, but not a professional linguist.