Mi ne scias, kiu venos al la kunveno.

Here kiu means who and introduces an indirect question:

• Mi ne scias = I do not know
• kiu venos = who will come

So kiu is not just a random relative word here; it is the question word inside the larger sentence.

Because kiu venos is an indirect question, not an ordinary that-clause.

Compare:

• Mi scias, ke li venos. = I know that he will come.
• Mi scias, kiu venos. = I know who will come.

You do not use ke before question words like kiu, kio, kiam, kie, kial, kiel when they introduce an indirect question.

So:

• Mi ne scias, kiu venos. = correct
• Mi ne scias, ke kiu venos. = incorrect

Because the coming happens after the moment of speaking, from the speaker’s point of view.

• venas = comes / is coming
• venis = came
• venos = will come

In this sentence, the speaker does not know which person will attend the meeting in the future, so venos is the natural choice.

Yes, kiu can mean either who or which one, depending on context.

In this sentence, because it refers to a person coming to the meeting, English normally translates it as who.

Examples:

• Kiu venos? = Who will come?
• Kiun libron vi volas? = Which book do you want?

So kiu is flexible, but here the context clearly points to who.

In Esperanto, ne normally comes directly before the thing it negates.

Here it negates scias, so:

• Mi ne scias = I do not know

That is the standard word order.

Mi scias ne would sound unnatural here and would not be normal Esperanto.

Here kiu is the subject of venos, so it stays in the basic form without -n.

• kiu venos = who will come

The person referred to by kiu is the one doing the action of coming.

If it were the direct object, then it would take -n:

• Mi ne scias, kiun vi invitis. = I do not know whom you invited.

So in your sentence:

• kiu = subject
• no -n

al means to, showing direction or destination.

• veni al la kunveno = to come to the meeting

This is the normal way to express attending or coming to an event.

You may also see expressions like:

• veni al la lernejo = come to the school
• iri al la festo = go to the party

So al la kunveno tells you where the person is coming to.

la is the definite article, meaning the.

So la kunveno means the meeting, probably a specific meeting that both speaker and listener can identify from context.

Compare:

• kunveno = a meeting / meeting in general
• la kunveno = the meeting

Esperanto uses la much like English uses the.

In Esperanto, a comma is often used before a subordinate clause, including indirect questions.

So the comma helps mark the break between:

• Mi ne scias
• kiu venos al la kunveno

It is very common and stylistically normal.

Yes.

• Mi ne scias, kiu venos. = I don’t know who will come.

Adding al la kunveno just makes the destination or event explicit:

• Mi ne scias, kiu venos al la kunveno. = I don’t know who will come to the meeting.

So the shorter version is grammatical; it just gives less information.

Yes, that is possible, but it is less simple and less direct.

• kiu venos al la kunveno = straightforward, natural
• kiu estas venonta al la kunveno = literally something like who is about to come / who is going to come

For most everyday situations, kiu venos is the better choice. Esperanto usually prefers the simpler finite verb form when it works well.

It contains an indirect question.

Direct question:

• Kiu venos al la kunveno? = Who will come to the meeting?

Indirect question:

• Mi ne scias, kiu venos al la kunveno. = I don’t know who will come to the meeting.

The second sentence reports the content of a question instead of asking it directly.

Yes. In this sentence, kiu is grammatically singular, but it can still mean which person out of several possible people.

The speaker is wondering about the identity of the person who will come, or about each person in an unknown set depending on context.

If you wanted to make plurality explicit, you could use kiuj:

• Mi ne scias, kiuj venos al la kunveno. = I don’t know who will come to the meeting / which people will come to the meeting

So:

• kiu = one person
• kiuj = multiple people