To be honest, it's a bit too vague to gauge in this way, especially when there's no mention of brand names as i believe that audio is all about equipment matching as well.

There's some question about your "method A" though. So basically you'll be using a 2-channel for the tweeters, a 4-channel for the front and rear components(this, i don't really understand as you already had a tweeter's amplifier, you are adding another component set, making it 2 pairs of tweeters in front? ), and a 2-channel or monoblock for the sub?

Theoritically, being able to run active(run active as in tweeters get their own amplification, and the front woofer/midbass, whatever you call it, would be getting their own amplification and you can ditch the passive crossovers) would be better in an in-car environment unless you have the expertise to make and fine-tune your own passive crossovers.

My suggestion, if you ain't driving a HUGE MPV or Larger sized sedan, just ditch amplifying the rear speakers or just get a small amp for it. In fact, if you haven't bought the equipment, might as well ditch the idea of getting new speakers for the rear and just use back the stock ones powered off the HU, concentrate the $$$ on the front stage and subwoofer instead.

This way, you could still get an electronic crossover(with bandpass function, which is a combination of highpass and lowpass, on the midrange/midbass), a 4-channel amplifier for the front components and a 2-channel or monoblock for the subwoofer.