There is a chance that I misinterpret something, but I hope this answer helps a little anyway.
First, I want to make sure that you know that cum is not only the preposition "with".
Here it is the conjunction "whereas", "while", "though".
The suggested translation contains two (almost) independent parts which are best parsed in isolation (at first):
- et Demosthenes et milites ob hanc cladem metu sunt commoti
- Nicias eam paene exspectasse videretur
In the first part I would read ob as "because" rather than "at".
Technically, the second clause is subordinate to the first, and that is why it is in conjunctive.
The second part is an nominativus cum infinitivo: "Nicias almost seems to have expected it".
It is a passivised version of a more familiar accusativus cum infinitivo:
Niciam eam paene expectasse video.
I see that Nicias has almost expected it.
So, Demosthenes and the soldiers are not treated indirectly.
The second clause is subordinate to the first one, and the NCI is subordinate to the verb videretur.
In particular, Demosthenes and the soldiers are treated together, but I do not quite see what you mean by Nicias being a third element.
Remember that the what you call an "answer" is actually more of a "suggested translation".
It is not the single correct way to phrase things, but of course it makes sense to try to understand how it works.