I think it’s the second one.
Offhand, I know the “head over heels” version is used in The Jungle Book (1894). In “The White Seal”, it appears in the text with Matkah “knocking the youngsters head over heels” with her flippers and possibly in variant form in the Lullabye as “Or your head will be sunk by your heels”. That suggests the inverted version was established by the end of the 19th Century at the latest. Various sources suggest that the original “heels over head” version came into use in the 14th Century, but I haven’t seen any cites of a first documented appearance. As Gyrate says, the inverted version likely came to be preferred because it’s easier to say.
As to its meaning, it’s pretty clearly a reference to somersaulting, with a connotation of tumbling out of control.
Seize the day.
So I’m supposed to live for today like it might be my last day on earth? OK. I guess I can always retire to the trailer park when I hit 70.
Really? I’ve always understood it as an English colloquial expression similar to schadenfreude.
ETA: That is, more like the third option.
Well…I guess that I did. 
Even more stranger: Ass over tea kettle.
He who hesitates is lost.
Look before you leap.
The pessimist in me parses it like “Bad things come in threes.” If you are miserable now, just wait; things could still get worse.
Lyrics to “Best of All Possible Worlds” from Bernstein’s musical Candide
Once one dismisses the rest of all possible worlds.
One finds that this is the best of all possible worlds
“I’ve just come off an unhappy love affair, so I don’t see why anyone else should have a good time!”
Or as Frank Burns said in an episode of MASH, “Stop laughing without me!”
“You betcha’ !” (You bet you!)
Does this mean I’m wagering myself? How could I possibly lose?
Unless I’m wagering my actual person into indentured servitude.
Huh. That never occurred to me before. Might it be a shortening of something like “You bet your ass” that avoids explicit cursing?
Of course, that doesn’t make a whole lotta sense either. The long version would have to be something like “I’m so sure of [something], you could bet your life on it and be sure you wouldn’t lose.”
If that’s the case, no wonder it became “you betcha”. 
Half again as much. Johnny L.A. used this in a recent thread and I thought it meant half, he said it meant 1.5 times something. I’d never heard the phrase before or if I had I never realized that’s what it meant. I asked some other people if they’d heard it and they hadn’t, I wonder if it’s a regional thing?
I think Johnny thought I was being snarky about it, but I wasn’t.
I don’t think it’s a regional thing. It’s pretty self explanatory. “I want half again as much”…you want that and half again.
My mom used to say this: They’d bitch if you hung 'em with a new rope.
I didn’t understand it then and I don’t understand it now. If you’re hanging someone, are they supposed to be happier because you’re using a new rope instead of an old one?
But schadenfreude is different in that it doesn’t require that the person himself be unhappy, just that he be unhappy at other’s misfortunes. You can be perfectly happy yourself and still feel schadenfreude when someone else slips on a banana peel.
Also, that link doesn’t suggest that the person who is miserable is actively making other people unhappy themselves; they want them to be unhappy for whatever reason.
“Misery loves company” just means that people who are unhappy prefer those who are around them to also be unhappy, because seeing happy people rubs in the fact that they’re unhappy.
I’m in the UK and I’ve always heard this as “half as much again” which seems to me to make the meaning of that much and another half clearer.
Wiki lists an interesting historyand says it was the first American fad to go national. Though they do caution at the beginning: “Although there are a number of stories suggesting the possible origin of the phrase, none has been universally accepted.”
“read for comprehension.” Everyone on the boards says it and I don’t know what it means. How else can one read???
I could care less
Cheap at half the price