Ages ago, I used to work in a car stereo store. Your questions are good ones, but they don't have cut and dried answers. I can give you my personal choices for most situations.
Generally I find any woofer bigger than an 8" to lean towards giving more volume, but less accuracy. Personally I like accuracy, others like more bass volume. So I would go for two 8's over one 10". I"d crossover the two 8's with an analog crossover between the two so that each one could be even more accurate.
Five channel amps are a compromise (less space taken up for more channels, and less installation time), and compromises are rarely best. Generally you will want a separate dedicated very high power amp to run your sub-woofers...unless you will be happy with like one 8" woofer in your system. Unfortunately, separate bass amps are usually physically large, and take up a lot of space, which you may not have to spare.