2. you want the most power for the front speakers. if you get a 4 channel amp (and you should), bridge it to the front speakers for maximum power... or run active. you want most of the sound coming from the front - giving the fronts much more power lets that happen easily. you also get the best performance. "rear fill" is helped when the rear speakers aren't equally loud as the front (there are other aspects to it as well).

5. look for a head unit with time alignment.

sorry, got cut short.

having crossovers in the head unit allows you go make adjustments from the driver's seat - listening to the changes, and even doing it on the road. they can supplement the amp crossovers, or add to them.

they won't replace the passive crossovers that come with the speakers unless you are powering each speaker (each tweeter and each woofer) separately. we call this running "active" meaning each individual speaker has it's own amplifier channel. with a 4 channel amp you can power the component tweeters from channels 1&2 and the component woofers from channels 3&4.

HOWEVER, you need a crossover in the 3-4kHz range, and most amps don't come with that. so you need a head unit that is designed for running active, meaning built-in crossovers. or, you can buy an external active crossover and split the front RCA output before the amp.

part of running active means your crossovers are electronic, before or in the amp. the coaxial speakers usually won't allow for this since the tweeter crossover is usually built-in. some speakers are component but allow for a coaxial mount of the tweeter. if it has an external passive crossover and you can choose where you want to mount the tweeter, we call it a "component set".

You have a lot going on for being new to car Audio
I commend your research and effort to educating your self

You are taking on a substantial project with a complete Car Audio make over
I didn’t see any reference to the budget for this project

My very first suggestion would be “Simplify”
Complicated or fancy headunits do offer a wide range of versatility and features
However theses controls, setting and features can only be useful with advanced experience and in-depth understanding. When you have a dozen or more settings or controls that can make your Audio System sound BAD and only a few of the right combination to sound good.

I just trying so tell you I deal with customers daily that push one or two button and put their whole system out of wack

You’re considering a two amplifier set up
A 500.1 and a 12” is a lot bass

I would recommend one multi channel amp 4-5 or 6 channels (a four is all I think you need)
One amp is easier to deal with and less expensive less space, wire, connections and labor

All you need is the settings on the amplifier (set once)

Looks like you will be spending an easy 3,000.00 on equipment and materials