I think there is a lot of good information in your post, if I can get my brain cells running fast enough to properly digest it!
OK, first off, what is "L-R" crossovers? Left/right?
Second, (and I'm trying to get my knowledge of acoustic theory up to speed at the same time, so be patient), I'm guessing you recommend 1st/4th/8th order crossover because it doesn't screw up the phase in the XO region, correct? Would that include that linkwitz-raleigh 24db/octave design (BTW, what order is that?).
Third, sealed with Q ~ .5 sounds a lot like what I had in mind. If others could offer some suggestions for appropriate driver/enclosure configurations, I would be most apprciative. I think I would know where to start with a sealed "sub" design, but having a higher frequency response I'm left a bit uninformed as to driver requirements. I'd also like to keep cost down, seeing as I would need four of these... probably avoid $200 drivers if possible.
Fourth, anyone have specific recommendations for a four channel active crossover with the requisite 1/4/8th order slope that could handle both low pass (for the mid-bass driver) and high pass (for the planar driver)? I would also consider sending the lowest information back up front to the sub (IB), so that low pass might actually work better as a mid pass band. Prosumer gear? I could see spending as much here as for all the drivers and materials required.