What the Hell does Popeye (and Bluto as well) see in Olive Oyl?
From what I can tell, she's a skinny bitch.
That's the joke.
At the time period, skinny people were generally considered the more attractive kind of people, weren't they? Also, Olive is the good kind of cute—not the tasteless, sappy cute you see in a Disney cartoon—she's obviously ugly, but goes about her business like a cutie—something that Famous Studios missed the point of and just made her generic cute. True cuteness has a level of ignorance to it. Also, Popeye ain't exactly a beauty himself.
Olive's body has changed shape over the years; in some of the Famous cartoons she's actually rather glamorous.
Olive was never intendend to be attractive in any way. She's ugly, fickle, flighty, and genrally unpleasant. She's markedly easier to take in the comic strip, which didn't rely on her frequent dalliance with Bluto (they don't, in fact, contain Bluto much at all)to set up the generic "Popeye eats spinach and beats the snot out of Bluto" plot. As the second troper suggests, her unattractiveness and unpleasantness is intended to be a joke, given how devoted Popeye is to her (though to be fair, he ain't much of a prize himself).
What does Olive see in THEM?
Oh come on now. Popeye is hot.
In the movie at least, Olive never actually liked Bluto and only got engaged to him because it protected her family from the tax collector. She liked Popeye because he was a Nice Guy who was adorably devoted to Swee'Pea.
The shorts, like most other theatrical cartoons, have no real canon and just have a set cast of characters that can be used to fill different roles, which is why Olive Oyle is so inconsistent in her attraction to Popeye and/or Bluto, similar to Minnie in the old Mickie Mouse shorts. Canonically speaking, she has no attraction to Bluto, seeing him as a Jerk Ass bully (which he is), and loves Popeye due to his bravery and devotion.
Why couldn't Popeye just eat the spinach outright instead of waiting for "the right moment" and therefore avoiding those times when Bluto becomes Genre Savvy?
Well, Popeye's favorite food is spinach, so he probably eats it all the time; in any given cartoon he probably had spinach for breakfast and something with spinach in it for lunch; if you watch the cartoons or read the comics you'll see that he is already very strong even before he boosts up on spinach. It's just that he has a can ready for when he needs an extra energy boost.
Also, it's limited how many cans of spinach he can carry on his person at any given time — one or two seems to be the limit. (Sometimes he's shown to carry more, but those instances are clearly just done for the sake of a gag.) Hence he has to save those extra energy boosts for when he really needs them.
In the comics (of recent vintage, anyway), Popeye breaking out the spinach is treated as a Let's Get Dangerous moment. Usually, he can handle the threat of the day without it. In the cartoons, note that Popeye is usually winning whatever contest he's into until the bad guy (not always Bluto) pulls a dirty trick, and Popeye pulls the spinach to even the odds.
Pretty much. Popeye usually doesn't need the Spinach, he is superhumanly strong as he is.
What happens if Popeye's opponent also eats spinach? Does a Yin-Yang Clash occur?
The opponent would just get stronger, and sort of negate the point of Popeye eating the spinach. Popeye's fed Bluto spinach during the occasional Enemy Mine episode and it worked the same for him.