Learn English – Do you need commas setting off a “when” clause in a sentence

This sentence has stilted and contrived syntax. It's very unpleasant to read. If you want this word order and all those confounding commas, then give it the structure of verse:

Roman sat,
quietly,
in his seat,
not giggling and chit-chatting,
like the other kids from his school.

The sentence is ambiguous, however. Too many commas (all those postterm pregnant pauses are unnecessary and undesirable, unless the sentence is in free verse) and the wrong word order.

It wants to say:

Unlike the other kids in his class, who were giggling and chit-chatting, Roman sat quietly in his seat.

So why not say:

Roman sat quietly in his seat, not giggling and chit-chatting like the other kids in his class.

or

Roman sat quietly in his seat, not giggling and chit-chatting, unlike the other kids in his class.

If you drop the contrasting phrase, you lose something, yes, but your sentence

b) Roman sat, quietly, in his seat, like the other kids from his school

incorrectly disambiguates the sentence. It has to be

b) Roman sat, quietly, in his seat, unlike the other kids from his school.

Then the meaning of the original isn't lost.

It should be omitted - the comma obscures the meaning.

The phases "indicated above" and "presented below" are complete phrases in themselves and should not include a comma.

In fact, with the comma, the sentence seems incomplete, and leaves the reader expecting something like:

Place the item as indicated, above the table [or above something else].

i.e. It has previously been indicated that the item should be placed above the table.

A correct example will be presented, below the diagram [or below something else].

i.e. The example is presented below the diagram.