Okay, he picked the second stupidest option available to him, he still
attacked her recklessly.  He stuck his foot in it, stupidly.  

        So defending the one he has feelings for, in the only way
he knows how, and considering that Shampoo had quite intended
to kill/seriously hurt Akane, is the second stupidest option available
to him.  I know if someone tried to physically hurt my fiancee I would 
not try to work things out in a calm, rational and non-violent manner. Would
you?

        And he did not really attack her after all.  He aimed for
the bonbori, to disable the weapon as it were, but never knew it
was going to go flying up and come straight down on Shampoo's
head.

Like I said,
I consider all the so called engagements to be meaningless.  Ranma had no
choice in any of them. 

        True.  Ranma was too little to argue.

Now, if you argue one or two are valid because his
father agreed to it then I argue back to say that has no legal basis even
in Japan

        True.  Assuming the law covers that aspect in Japan, which
I assume it does because I have no idea about the laws in Japan.  Still,
what about affiancing?  What does the law in Japan say about that?
Does the fiance, or fiancee, have to consent to it?  (I don't know)

whereas Shampoo's does in her society (not in Japan, granted) and
carelessness and even ignorance is not a defense (given that he was at >one
time in the Amazon society and had the obligation to respect their laws >and
therefore know them.  
If this was the first time he met an Amazon I'd let
him off but not under the actual circumstances.)

        Ranma-chan had the obligation to know the laws of Joketsuzoku,
and therefore should have known that challenging the Champion
and winning would have dire consequences for her.  Following
your argument, ignorance/carelessness would not have been sufficient
defenses.  If she had only asked, a lot of trouble would have been
averted.

        Again, Shampoo had the obligation to respect the laws
of Japan even though she didn't know that killing is forbidden.
Likewise, since she had the obligation to respect Japan's laws,
she should have found out whether killing is allowed in Japan
or not (an absurd notion, granted) and arrange her travel plans
accordingly.  Therefore, Shampoo shouldn't even have come to
Japan in the first place if she had rationalized it out.

        But we know she had not been rational in following Ranma
to Japan in order to stick a spear into him/her.  Her honor and
pride had demanded it.  

        Likewise, if Ranma had been rational, he would have asked
Shampoo about the laws regarding men defeating Amazons.  If he
had he might not have made an attack at Shampoo - which was really
directed at the bonbori - and he would have been saved a lot of
trouble.

        But we know Ranma, of all people, is not rational and 
considering that Shampoo tried to kill/hurt Akane, and in the
heat of the moment, his concern for Akane and his anger at
Shampoo had demanded his actions.

        All in all, if everyone had been rational, Shampoo would
not have come to Japan looking to kill Ranma-chan, and everything
thereon regarding Shampoo would not have happened.