Learn English – What does “fall out of heaven” mean

"Stick your fork in it and see if it's done."

To stick a fork into something can have a negative meaning, along the lines of 'I don't like it, it's terrible, it's not good', or, per definition 2 of the link, 'To be completely destroyed or defeated'. Or as in The Urban Dictionary (TUD) entry 3 'Indicating a losing or lost cause'.

So, being music and song, when lines can have many meanings, it seems a play on words of both "a fork in the road" as a decision or "another turning point" and to just stick a fork in the road, because I think the situation (or my life) is terrible or even a lost cause, and/or it shows that my attitude toward the turning point in life, or all of life, is negative.

But, as always, poetry and song lyrics are open to multiple interpretations, some of which may be unintended or even unknown to the writer(s).

In addition to the uses you found we also use the word "something" to describe a thing that we can't remember or don't know.

For example, "I can't exactly remember what Alice said. She said something about 'the furious five' and 'alive.'"

This is a very typical use, but not exactly like the one in the song. In the song, "something" doesn't describe the forgotten word, it stands in for the forgotten word:

"Protect the valley, something (some word I can't remember), something (some word I can't remember), something (some word I can't remember), something (some word I can't remember) alive"

This is a common* way of singing a song, or quoting a speech, that you only partially remember. It's not formal, so don't use it in an essay for school, or a published article.

*The joke here is that we might expect that a viewer of a cartoon show would sing the theme song like this, because they could only remember some of the words. We would not expect the person singing the theme song on the show itself to have forgotten the words.