Where to insert comma(s)?

I usually follow Strunk & White and only deviate from their rules if strict adherence would result in something that breaks the flow of reading/thought.

• Enclose parenthetic expressions between commas.

• Place a comma before and or but introducing an independent clause.

• Do not join independent clauses by a comma.

This points to number 3 being the correct sentence. As for the last two, I definitely prefer number 7 as well. However:

She tried, and she failed. 

And finally

She tried; she failed. 

As commonplace as it may sound, I would add: never judge any sentence as it stands by itself. Depending on the mood and style you want to set, anything might be OK in the right context.

I'm always struggling with over-comma-fication, meaning my own inclination to put in too many. [en- and em-dashes are one (lazy) way around this proclivity, as is paying close attention to how the expository pieces in The New Yorker are punctuated.]

I'd choose sentence #3 in your first sampling. It segregates "as expected" (which is incidental to the main event) but otherwise keeps tried and and together, drawing the reader forward and so letting the sentence flow.

By the same token, I think sentence #7 in the second set is preferable. It's a straight-forward exposition wherein the comma's extraneous. If you really wanted to build up tension, use ellipses or an en-dash -before or after the "and"- to signal as much instead.

In regards to example 6, the comma would be more appropriate if the next word was "but":

She tried, but failed.

In your example "She tried and failed", if the sentence continues after the "and failed" you very often see parentheses around the "and failed" also:

She tried (and failed) to...