I agree that if it's a strong beer you shouldn't have any problem just throwing the chips in there. If you really want a little punch soak the oak chips in good bourbon for a few days then put the oak and bourbon in the secondary. As far as your second question, don't worry about the extra headspace in the carboy. The last bit of fermentation that happened in secondary created enough co2 to put a "blanket" of co2 over the surface of the beer keeping o2 out.
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