crossover use and speaker placement

1.  No.  The Extremis won't beam within that frequency range, so any aiming will more than likely be a waste of time.

2.  It will probably work.  Just keep in mind that the quality of the crossovers in those things aren't usually the best.  The potentiometers in amplifiers are usually pretty inacurate and might be difficult to dial in correctly.  And from what I understand, the crossovers in head units aren't usually very good either. 

As a suggestion though, I recommend taking the Extremis down much further; maybe to around 50 Hz and run the sub below that.  This should give you a "bass up front" sound. 

Good luck!

First, the new Alpine amp is not stable at 2-ohms bridged.  If you want to use the rear channels for mid-bass, you must use at least 2-ohms per channel.  Plus mid-bass is stereo, not mono, so if you use Extremis woofers for mid-bass (a good idea, by the way) they should be mounted in stereo such as in front kick-panels or in the doors and operated with a stereo feed, but not "aimed."  The Alpine ICE amp puts out 150 watts per channel into 4-ohms which is about perfect for the Extremis, so get the 4-ohm versions and use one on each channel 3 and 4.  Next, there is nothing wrong with trying your "bandpass" crossover idea for the mid-woofers.  It might be tricky to get set up correctly, but if the amp has a LP that will reach as high as 400Hz, go for it.

-edit- and I see Steven chimed in while I was typing.  I agree with him, 100Hz HP on an Extremis is not utilizing its full potential.  Indeed, I know at least one person who is using one AS a sub and it kicks ass ported at 28Hz.

Are you going to have any other speakers in the back?  By themselves, I don't think they would do much good except for maybe detract from your front stage, especially without any level control on it.  They will do a great job on the midbass, much better and much lower than most other midbass drivers on the market.  I say try all 3 crossover points and choose which one sounds best to you. 

The Zapco will more than likely be a MUCH better amp with much more accurate signal processing.  Especially if it is in the digital range.  The Alpine amps look nice, but I am somewhat leary of their power ratings, especially over time.  That's a lot of power to put in such a small package without having some compromise in performance.

Wow, you bought Kit LCR's to install in your car?  Well... suffice it to say I would not have recommended that.  :)  Did you actually buy the kits with the crossovers, etc. or did you simply buy the drivers?