Neniu el miaj geamikoj povis veni hodiaŭ, ĉar ĉiuj laboras.

Neniu usually works as a singular word meaning no one / nobody / not a single one.

In this sentence, the subject is neniu, so Esperanto treats it as singular:

• Neniu ... povis = No one ... could

Even though the idea involves many friends, the grammar focuses on not a single person from that group.

You may sometimes see neniuj in other contexts, but here neniu is the normal choice.

Because the grammatical subject is neniu, not geamikoj.

So the structure is:

• Neniu ... povis veni = No one could come

Geamikoj is inside the phrase el miaj geamikoj and does not control the verb. The verb agrees with neniu, which is treated as singular.

Here el means out of / from among.

So:

• neniu el miaj geamikoj = none of my friends
• literally: no one out of my friends

This is a very common Esperanto pattern:

• unu el ili = one of them
• kelkaj el la libroj = some of the books
• neniu el ni = none of us

A native English speaker may expect a direct equivalent of English of, but in this partitive sense Esperanto normally uses el.

Amikoj means friends.
The prefix ge- shows a mixed-sex group: male and female together.

So:

• amikoj = friends
• geamikoj = friends of both sexes, or a mixed group of friends

In modern usage, some speakers use amikoj in a general sense anyway, but geamikoj makes the mixed group explicit.

Because miaj describes a plural noun: geamikoj.

In Esperanto, adjectives and possessive words like mia, via, lia behave like adjectives and agree with the noun in number (and case, if needed).

So:

• mia amiko = my friend
• miaj amikoj = my friends

That is why we get:

• miaj geamikoj

Because geamikoj is not the direct object here.

In the phrase:

• neniu el miaj geamikoj

the word geamikoj comes after el, and nouns after prepositions normally do not take -n.

Also, this whole phrase is part of the subject, not the object.

The direct object idea does not appear here at all.
Compare:

• Mi vidas miajn geamikojn. = I see my friends.
  Here geamikojn gets -n because it is the direct object.

This is a very common Esperanto structure.

• povis = could / was able to
• veni = to come

After a modal-like verb such as povi (can, be able to), the next verb stays in the infinitive:

• Mi povas veni. = I can come.
• Li povis helpi. = He could help.

So:

• Neniu ... povis veni = No one could come

Only the first verb is conjugated; the second stays in the basic -i form.

This is a very natural tense choice.

• povis veni hodiaŭ = could come today / was able to come today
• ĉar ĉiuj laboras = because everyone is working / because everyone works

The first part talks about the result for today: they were not able to come.
The second part gives the reason, which is presented as a current fact: they are working.

Esperanto often uses the tense that fits each clause individually, just like English:

• He couldn’t come because he is sick.
• They couldn’t join us because they are working.

So the tense difference is normal.

Because ĉiuj means all (of them), everyone as a plural idea.

Compare:

• ĉiu = each one, every individual one
• ĉiuj = all, everyone

In this sentence:

• ĉiuj laboras = everyone is working / they all work

A native English speaker should notice that Esperanto often distinguishes:

• ĉiu homo = each person / every person
• ĉiuj homoj = all people
• ĉiuj = everyone / all of them

It refers back to miaj geamikoj.

So the meaning is:

• None of my friends could come today, because all of them are working.

Esperanto often leaves out a repeated noun when the meaning is clear:

• ĉiuj laboras instead of ĉiuj geamikoj laboras

That sounds natural and avoids repetition.

Ĉar introduces a full clause with its own verb:

• ĉar ĉiuj laboras = because everyone is working

That is why ĉar is the right choice here.

Pro is a preposition meaning because of / due to, and it must be followed by a noun phrase, not a full finite clause.

Correct examples:

• Li forestas pro laboro. = He is absent because of work.
• Neniu povis veni pro laboro. = No one could come because of work.

But in your sentence we have a complete clause:

• ĉiuj laboras

So ĉar is the natural option.

Yes. Esperanto word order is fairly flexible, as long as the meaning stays clear.

The original:

• Neniu el miaj geamikoj povis veni hodiaŭ...

Other possible placements:

• Hodiaŭ neniu el miaj geamikoj povis veni...
• Neniu el miaj geamikoj hodiaŭ povis veni...

The most natural position depends on what you want to emphasize:

• Hodiaŭ first = emphasis on today
• after veni = a more neutral flow

Because ĉar introduces a subordinate clause:

• ĉar ĉiuj laboras

In Esperanto, it is very common to put a comma before a clause introduced by words like ĉar, ke, kvankam, se, and so on.

So the comma helps show the structure:

• main clause: Neniu el miaj geamikoj povis veni hodiaŭ
• reason clause: ĉar ĉiuj laboras

Yes.

• Neniu el miaj geamikoj povis veni focuses on not a single one
• Miaj geamikoj ne povis veni means my friends could not come

The second version can sound like the whole group as a unit could not come.
The first version more explicitly stresses that zero people from that group came.

So neniu el miaj geamikoj is especially good when you want to highlight none of them at all.