The stock woofer is 4 ohms, the tweeter is 6. They are run in parallel creating a 2.4 ohm load. Formula for parallel is 1/R1 + 1/R2 = 1/Rt
If you're going to be running aftermarket speakers, hook into the power where the connector from the car connects to the woofer. You won't have any problems with that as the amp will see the load as what your speakers are rated for.
Its actually a capacitor, not a bass blocker, but it does act as a "bass blocker" based on the impedance of the speaker.
There are no speakers that will fit in a car that can handle 300 watts. Are you running something like this? Eminence - The Art and Science of Sound
If not, then you're wasting your time trying to give those speakers 300 watts. Also, the amp determines the amount of power being delivered so the stock amp will only continue to deliver the same amount of power it always could. Only changes based on impedance.
Also wrong.Only speakers with a VERY low inductance will have a curve thats even CLOSE to flat impedance. Also in speaker design, for example, if a tweeter has an impedance peak of less than 10 ohms, it will sound over damped and generally pretty crappy. Impedance peeks are a necessary evil of speakers.
Consider a subwoofer in a vented box, the port will end up creating another impedance peak in the curve. (technically, maybe not in that order but you get the idea.)
Its the lowest point in the impedance curve you should really consider the most, especially if the music you're going to be listening to has a lot of content in that range.
Another note: Amps are voltage based devices so don't worry about the peaks in the impedance curve, you won't magically have less output there.