In this case even means approximately ‘what is more, moreover’.
Even doesn’t fit well in the first part of the clause, where really implies ‘in contrast to’ the previous clause, while even implies ‘to a greater degree’—the two adverbs seem to contradict each other. It does fit well in the second part, where it lifts general reference to the even higher level of common knowledge.
The sequence may be paraphrased
I am closing this as “Duplicate”, which it is
BUT
it is actually “General Reference”, too—
AND NOT JUST
ordinary “General Reference”
BUT
“Common Knowledge”,
a fact so widely known that any native speaker could be your reference.
[III] - I ate the pie, and since then I have had a stomach ache.
I think this is the most natural comma placement: it divides the two independent clauses, which is what commas do.
I don't know who wrote your Sentence I (SI). To me, putting the two commas here is unnatural. It represents to me an unnatural way of expression. It also means there are two pauses within five words and two pauses in a straightforward sentence of 13 words. I don't know why anyone would want to slow down that much.
The two commas in SII make sense, because they separate a unitary phrase (since then) from the rest. But although this two-comma version works much better than SI, it's not as eloquently simple and natural as SIII. This is because SII also introduces an unneeded double pause in a thirteen-word sentence that needs, at most, one pause. (Note that my use of commas to separate at most is similar to II's comma use.)
You could also dispense with the 'since then' and write the remaining with either no comma or one comma:
[SIV] I ate the pie and I have had a stomach ache.
To me this expresses both thoughts as one unit containing two facts. It does not really stress a causal or resultative relationship between the two actions/facts. Also a comma is just plain unnecessary because now the sentence is unencumbered by the 'since then', and it has only eleven words. I mean it is short and consists of two short independent clauses.
[SV] I ate the pie, and I have a stomach ache.
This goes back to the simple connection of the two independent clauses with a comma immediately after the conjunction, as in SIII. This is very frequently done, and at least helps the reader parse the sentence if not also slow down a tiny bit. (See how I used only one comma in that last sentence, and only to separate the two clauses? Oops, I just did it again.) SV probably does not present the two actions in terms of one unit of dual-facts like SIV. Perhaps because the pause also allows a millisecond for the reader to make some causal or resultative assumption between the two clauses--even though the comma by itself does not do that.
Best Answer
[Note: It should be "I didn't DO it" (not TO it)]
[Note: At least in American English, it sounds a bit unnatural to end the sentence with "do." We would be more likely to say "I didn't do it as he'd asked us to." and leave the "do it" implied. Yes, that ends the sentence with a preposition - most American English speakers don't care. Or, if you are going to include the "do" write out the whole thing: "I didn't do it as he'd asked us to do it." This emphasizes that you actually did the job, but not in the specific manner that it was asked for.
"in the way" and "the way" are equivalent and mean that I did not complete the order to his specifications; I did something, but not what he asked.
"I didn't do it as he'd asked us to do" could mean the same thing, but could also mean that I simply didn't do it at all. The meaning is more vague and more dependent on context. "As" is a slippery word, prone to multiple meanings. There is even the unlikely possibility that you are saying you would have done it for anyone else but him, but because he asked you refused. "I didn't do it, as HE'd asked us to do."