<-How Rei actually gets into Ranma's body.>

A couple of thoughts:
Assume Rei's mental patterns were 'imprinted' by the explosion/AT field
and the imprint was deposited randomly into Ranma's brain/mental
configuration. Additionally you could use this as an additional plot
mover, linking the curse's presence to that of Rei's, as well as
bringing up the potential of random shifts in who's in charge of who's
body (or in this case shape.)

<-How Rei's analytical and emotionless (but not for long :>) self
creates conflicts/resolutions for Ranma's current personality and/or
romantic engagements.>

Personally I think that she might be able to solve some of his romantic
problems (yikes, going to Rei for relationship advice) by looking at
them rationally, which appears to be against Ranma's M.O. Its also going
to cause some confusion and a good deal of alarm.

<-Action sequences (fight scenes) in which Rei is the primary element.>

Rei's fighting style would be the opposite of most of the Ranma
characters: businesslike, unspectacular but rather effective once she
gets the hang of it. The change in style would probably benifit her
performance, simply because of the difference in what they're expecting.
(A fact learned after playing a lot of Tekken, very seriously)

<-Details to the scene where Rei self-destructs her Eva to destroy the 
angel.>

It goes like this (essentially): Rei is sortied against the angel(16th?)
which rapidly shifts from a defensive to offensive posture. Asuka is
sortied as well, but fails to synch with her Eva and is withdrawn.
Shinji is launched after Rei begins to be contaminated by the angel
(those gross bulges that run through her eva and her). the Angel asks
her if she wants to be united with Shinji, which Rei doesn't quite
answer. After the Angel goes for Shinji, Rei reverses her AT field,
sucks the Angel in and destroys/overloads the Eva's core, taking out
most of Tokyo 3 in the process. (there are additional details revealed
in the revamped episodes, but I haven't seen them). Anyone with greater
knowledge of the scene (its been awhile) can probably add some more
info.

Jerico
"It never hurts to help" Eek the Cat