Nonessential Commas

Can somebody help to clear up three issues I'm having with 'nonessential commas'.

Firstly, does this use of the comma only concern itself with modifiers, wether it be appositive phrases, adjectival clauses, etc., etc.? They cannot be used to set aside other sentence elements?

Secondly, why can full clauses--clauses which could stand as their own sentences-- be enclosed in dashes and parentheses but not commas? Commas, dashes, and parentheses all function in the same way when 'setting aside parenthetical elements', aren't they? It would just become a simple splice with commas, no? 

And, finally, how can a nonessential aside have its own internal aside within one sentence? I'm talking specifically about commas here; I'm not talking about, for example, a non-essential relative clause that contains further context in parentheses or dashes. Here's an example sentence I've just made up:    "Jack opened the can with ferocity, which surprised his mother, who herself felt the same rage within herself." This is a terrible example (I've had a long day), but I come across sentences of this form all the time. There's even sentences that contain a parenthetical within a parenthetical, within yet another parenthetical. What's the deal?

Thanks

Commas are part of the mechanics of English text and are governed by the conventions set by your manual of style, either the one you've adopted or the one thrust upon you. These manuals differ in many respects, although they all agree on certain basic requirements. There is usually plenty of room for personal preference and style, although these dispositions are not plenary. That said.

Firstly, does this use of the comma only concern itself with modifiers....?

Commas concern themselves with more than modifiers, e.g., separating conjoined independent clauses, separating items in a list, or setting off parentheticals. If you're talking about nonessential and essential constructs, then by definition you're talking about modifiers.

Secondly, why can full clauses--clauses which could stand as their own sentences-- be enclosed in dashes and parentheses but not commas?

But independent clauses can be set off by commas, as in the following example with three clauses:

I came to Gaul, I saw my battlefield opportunities, and I defeated the natives.

If you're talking about parenthetical clauses, then dashes or parentheses would be preferred over commas to avoid collision the the usage above:

I came to Gaul, I saw my battlefield opportunities—And there were, believe me, plenty of those— and I defeated the natives.

Using commas instead of dashes might mislead your reader into thinking there were six things in the series instead of three.

And, finally, how can a nonessential aside have its own internal aside within one sentence?

Because as you scan rightward in the text, English is free to recursively add constructs deeper in its syntactic tree. That is, English can modify modifiers of modifiers of modifiers or digress from digressions from digressions.