Yup, that's an option too.
Yes, spot on for the 12NDL76 pairs, 2 channels needed (one per box) of 1600W at whatever impedance you settle on for maximum headroom or marginally less as constrained by budget. Bear in mind that most amp designs are capable of less output as Impedance rises, so an amp rated at 1600W @ 16Ω would traditionally be rated 3200W @ 8Ω. That's why series wiring for your mids is likely to get expensive, fast. Granted some modern amps are sophisticated enough to manipulate their output voltage & current to overcome this, including the Linea's you referenced, so if you're still considering this manufacturer you'd best contact them for actual 16Ω ratings.
Or just wire in parallel for 4Ω anyway, making the issue moot 
.
No for the DCX's though. What a passive crossover does is split the incoming signal from the amp to the 2 drivers, so at any given frequency only one is operating (slight oversimplification, but it's what's relevant here). That means that you'd only need to choose amp channels rated for the larger part of the pair, ie 220W @ 16Ω to match the program rating of the mid part of the DCX. Conventionally, that means you're looking for amps rated 440W @ 8Ω if you want to power them with a separate amp than your low-mids.
1) There would be nothing to gain by wiring both DCX's in series IMO. Firstly, that doubles the impedance to 32Ω nominal, meaning you need to start with a bigger amp to power them; you'd lose the ability to discern stereo, and you'd need a custom patch panel/wiring setup to make it happen. If you were really desperate to save one amp channel, again wiring in parallel to make it an 8Ω load would probably be more practical.
2) Yes, that's exactly how bridging works. Bear in mind though that operating any amp at it's minimum rated impedance as standard practice is not recommended.
Firstly, it means the amp is working harder the whole time, so may have a shorter working lifespan than a larger amp worked less hard. Second, it limits your options to get through a gig if something goes wrong. If you buy an amp capable of running the 12's at 4Ω on one channel, then if a channel dies you still have the option of pairing up both your low mid sections on one remaining channel. You do lose stereo, and a bit of total power, but not too much. If you had bought a smaller amp to bridge in the first place and then had to try and run all four 12's off a single channel to limp through the rest of a gig. you would effectively lose a lot more power.
3 & 4) I think I've covered these above.
Cheers,
David.
