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Yes, this is called pot in pot. This is a wonderful way to get the best of both worlds. You will bury the same size pot, in the ground, as your plants container size. That way you can take the pot in and out with no problem.

Each plant produces a spread of 3-4 feet when full sized. Most trees and shrubs produce root systems that are 2 to 4 times wider than the aboveground spread depending on conditions, so that would be 6 to 16 feet. But the more distant roots are likely to be less damaging and barberry roots are shallow and not known to be damaging to pipes. So Ithink 6-8 feet would be safe.

Barberry can take hard pruning. It's possible to cut it all down to several inches: it will come back. A less drastic approach is to cut 1/3 of the oldest (widest) branches to the ground every year. This keeps it looking good and helps contain how tall it grows. Additional trimming to maintain desired height can be done anytime.

Yes. These are deciduous shrubs, and this is normal.

Here is an article for more information: https://www.gardeningknowhow.com/ornamental/shrubs/barberry/barberry-shrub-care-tips.htm

Yes, it is certainly possible to mix different species in creating a hedge/hedgerow. The two species may have different growth rates, so they may be better suited for an informal hedge. If you want to keep the sides of the two species even with each other to make a boxy hedge, this will take more maintenance.
http://extension.oregonstate.edu/gardening/node/998

Barberry is one of deers' favorite foods. The deer tick is linked to Lyme disease. You may not mind thorns but barberry thorns are close together and sharp! Wear leather gloves. Here is a list of deer resistant plants. It is for my state so look up the list at your state's extension service. http://www.ipm.msu.edu/uploads/files/deer_resistant_plants.pdf