Seemingly Contradictory Comma Rules: Please help me.

Exclusions of a comma where I would otherwise feel inclined to include one: I've presented excerpts from various sentences in which I've encountered such examples whilst reading.

1. NO COMMA AFTER THE ADVERB (USUALLY) AS AN INTRODUCTORY PHRASE?

Example: I feel inclined to begin such a sentence like this: USUALLY + COMMA... Please supply me with examples and explications why the writer would omit the comma following the adverb usually, (particularly concerning contexts of an introductory phrase or clause that is begun with the word usually.) Why would the comma be omitted? Please provide me examples of sentences where the comma is utilized VS. those examples where it is not.

2. Why is there a comma after "sometimes" in the following sentence?

"My husband does the cooking, sometimes." This makes absolutely no sense to me, as this excerpt was taken from an English grammar text. Personally, I would never dream of INCLUDING a comma preceding the word sometimes in that sentence.

3. No comma after "in fact" or "for example?" I've observed numerous sentences where "in fact" and "for example" begin a sentence and there is NO COMMA following that phrase. This makes absolutely no sense to me and seems contradictory when considering the rules of basic punctuation.

Thus, I always include a comma when beginning a sentence with (for example, <---), and (in fact, <--)!

Here is another similar excerpt from a grammar book: "In fact we want them to be critical readers, ..."

It doesn't make sense as to why there isn't a comma immediately proceeding the introductory phrase "In fact"

4. No comma after a prepositional phrase that is presented as an introduction to a sentence?! This is frustrating! Examples of excerpts from an English grammar book:

"On the day after that she was involved with several tasks-- Why isn't there a comma after THAT?

Such as, "On the day after that, she was XXX XXX XXX. I would have placed a comma after THAT.

5. "On the next day she did not appear to be very tired at all--" The word day is a noun, followed by the subjective pronoun "she." Supposedly, we are supposed to separate nouns with a comma.

Shouldn't the above sentence be written like, "On the next day, <--- (COMMA), she was involved in XXX?!

6. No comma after THEN as an introductory word?! What?!

"Then for three dreadful days she did not appear at all." ------------ Why isn't there a comma placed immediately after the word THEN? Such as: "Then, for three dreadful days?!"

7. "In making my choices I have concentrated on those who-"

Why is there is no comma separating "In making my choices" from "I have concentrated on those who..." ?!

8. "From all this you will gather that I believe....blah, blah, blah-"
Why is there no comma immediately following the introductory prepositional phrase "From all this ?! Shouldn't there be a comma separating the phrase "from all this" ---> from "you will gather that I believe..." ?! Please grammatically analyze this sentence structure in such a way that I can comprehend the justification of its punctuation.

9. "In selecting the entries I have kept in mind the great amount of subject knowledge teachers need."

Again, why isn't there a comma separating the phrase "In selecting the entries" ...from "I have kept in the great..."

As in, "In selecting the entries," <--- COMMA, "I have kept in mind the great amount of subject knowledge teachers need."

ALL OF THE EXCERPTS PROVIDED WERE EXTRACTED FROM A CAMBRIDGE ENGLISH GRAMMAR TEXT.

Please help me by submitting detailed answers. Similar sentences as comparative examples would greatly assist my understanding, not merely just quotes from English texts concerning punctuation rules. I have amassed many English grammar manuals and have studied the comma rules quite thoroughly; yet I am still quite confused.

The comma rules seem to be exceedingly contradictory to an ludicrous degree. This is quite frustrating.

Thanks,

some guy