She not only causes him trouble herself, but she's also brought along
other members of her tribe to add to his misery.  (Including a rival for
her affections who can never quite defeat her "husband".)

Huh?  Rei and Ataru don't ever really fight.  Whenever they even come close, Rei
just sits on Ataru.

But, despite all of his denials and his involvement with other girls,
when the cards are down, he'll protect her.

There's a major difference; when Ataru thinks that Lum has left him (to go home 
or whatever), he gets depressed and tries to chase after her.  I'm not talking 
about kidnapping, I'm talking about Lum leaving voluntarily.  (I don't know if 
Viz translated any of these stories, but there are a a bunch scattered here and 
there).  It doesn't seem that Ranma would really care if Shampoo up and left.

In her desperation to keep her "husband", she uses bizarre schemes and
devices.

There's a big difference of degree, I think; further, that side of Lum gets 
really toned down as you move further into the series.  (I like the stories 
better once you hit volume 8 or so anyway, so I'll consider them more 
authoritative :-) ).

So, why is it the consensus opinion of this list that the Ataru & Lum
match is inevitable but the Ranma & Shampoo match is highly unlikely?

Basically because the author (Takahashi) bends over backwards to imply that this
is the case.

Gavin