Explanation Required!

Can somebody explain how the commas are being used in the following sentence:

'So it is allowable to use ellipses to indicate pauses or breaks in the writer's train of thought, as you see so frequently done in email, especially where a break is meant to feel uncertain.'
The sentence can be found in GrammarGirl's ''Punch Up Your Punctuation'', where, somewhat ironically, she doesn't delve into much detail about comma usage.

From my own understanding, the first comma signals that the phrase afterwards is supplementary (n0nessential) to the meaning of the main clause, but then the other descriptive clause appears after the second comma, as if it's a nonessential element to the first 'nonessential.' Am I interpreting this all wrong? One relative clause has its own intertwined clause within itself--like an onion-type formation--that is nonessential to the nonessential (phew!) ???

My intuition about this sentence being right, doesn't this mean one could in theory cast a sentence of this form off into perpetuity, with a never-ending string of nonessential clauses ''comma-ed off'' one after another?

I apologise for what might seem to some a stupid question, but I'm desperately trying to get my head around the more complicated uses of the comma!

If anybody has any suggestions for more technical sources that go into the depth I'm looking for, I'd be grateful. And thank you in advance to anyone who provides answers.

Sean