Correction. Ranma would do whatever he could to best Nabiki,

outside

the realm of martial arts. He would not attack her, nor would he

have

someone else attack her. 

Including foiling her plots, and when the interested parties

discover 

her machinations, things will get ugly.

A side effect of Ranma's attempt to foil Nabiki's plot. It has
nothing to do with any direct action against Nabiki by him, and
Nabiki is usually not on the receiving end of the consequential
damages.

If Bugs Bunny told Ryouga that walking off a plank would help him
defeat Ranma and/or win Akane, he'd do it. Ryouga is gullible.

He wouldn't and he isn't. He's at least as quick mentally as Ranma,

he's 

just afflicted with an intense boyish attraction to love--or, I

suppose 

you could call it, innocent lust. He goes wit-wanering when it

comes to 

matters of the heart, as do many Takahashi characters. Each and

every 

time that I can recall Ranma succeeding in tricking Ryouga, he had 
appealed to that weakness.

No, he appealed to Ryouga's loneliness, of which that is a side
effect. Ryouga is a loner, and as such doesn't receive much attention
or affection, especially from the opposite sex. Which is why when a
female becomes affectionate towards him, he gets flustered and drops
his defenses.

Ryouga *has* been manipulated, BTW. Although Cologne's true

motives

aren't certain (was she trying to get Ranma unhooked from Akane,

or

trying to make Ranma a better martial artist, or both? We'll never
know for sure), she *did* manipulate Ryouga into learning the

Bakusai

Tenketsu and fighting Ranma when she knew that the only thing the
technique would do is make Ryouga nigh-invulnerable to Ranma's
attacks. 

Cologne didn't make Ryouga do anything; she offered him something

he 

wanted, and he took it; it's immaterial that Cologne had an

interest in 

seeing Ryouga becoming stronger--that is what's called doing

business.

You seem to be confused as to the definition of manipulation.

Manipulation does not mean you force somebody to do something you
want them to.

In Cologne's case, the manipulation was very subtle. 

In fact, when Cologne first offered to train Ryouga, I think his

exact 

words were: "I'm not so weak I need to be trained by you!"

And yet, Cologne knew that as soon as Ranma kicked Ryouga's arse,
he'd eventually come around looking for her.

Ryouga is stand-offish and can be downright rude. He isn't this way

conciously, it's just a part of Takahashi's cultural satire. 

I'll give you that; it more or less depends on who he's talking to.

Yes, Ryouga is despressed, but he's still a warrior of bushido, and

he 

sees himself above normal people. Because Ranma is also a warrior

of 

bushido that he was compelled to keep Ryouga secret from Akane.

Ryouga 

is constantly using speech, phrases and mannerisms that show that

he 

considers himself a warrior elite--again, this is not something he

does 

conciously, it's just an archtype of Japanese culture.

I'll give you Ryouga's status as a warrior, I'll give you the bushido
angle, but the ego? Not likely. Cough up some evidence of this ego.