Tweeters in the kicks can and does work VERY well in alot of cars. To answer your questions.
1.Good imaging in the midbass and midrange frequencies is about PATH LENGTH DISTANCE, that's what time aligment attempts to correct, but the closer you can get it physically, the better. If you measure from the seats, kickpanels are a fairly equidistant spot. Also kickpanels are deep into the car, which helps with stage depth as well as less rattles, if you put a midbass down there. I know personally, I'll probably never do another door midbass, ever.
2.Tweets down low avoid early reflections off the glass and get early reflections off much softer surfaces. This will actually improve your image accuracy. Stage height at upper frequencies is somewhat a frequency response thing. Tweets in the kicks have better frequency response usually and the stage can be quite high. Also since the mid and tweet are together, you get better phase response at the crossover point.
And to OP: GO ACTIVE. You'll find very quick, especially if you experiment with raw drivers that car audio speakers are so overpriced it's ridiculous. The price people pay for Focal, CDT, MB Quartz, DLS, Rainbow, etc, etc is really crazy when you figure out how "high end" many of the speaker really are. 1/4 the price seems to be in the crossovers and another 1/4th seems to go to the name. $1000 speaker sets can easily be matched or beaten with $500 worth in raw drivers. If you a high effeciency guy like me, then nothing in car audio is of any real worth other than maybe the higher end Image Dynamics XS speakers, even then for MSRP, no thanks.. I got my mids used for about what the drivers were worth to me, 250, had I paid the retail for the full components, again, I would have been REALLY mad at myself once I went active and tried real pro audio. Even the $50 pyle pro's I had were just about as loud as the XS and had equal midbass performance, they just lacked midrange clarity, the XS were well rounded. I liked their midbass maybe a bit more than the BMS 8's I used after that, but overall volume and midrange realism goes to the BMS. Anyway, that's just some of my experience with going active and the freedom to explore that it brings, you.. It's worth it.