1) What exactly is this "cutting-edge modularity" CIG speaks of in the 600i and what tangible benefit will this have?
The 600i can swap out the luxury suite for an exploration package, but it's not a BUK. You buy one or the other ship.
2) What is the benefit of having two scanners on the 600i vs one on the Aquila?
I don't think it has 2 scanners. It has two scan stations. We don't know what the difference is here yet, but we know there are three kinds of scanning and one supposes it can do at least 2 at once. However you should note the scan stations are not on the bridge of the 600i, so if you're scanning alone, no one is minding the store. Of course if you're scanning you won't be surprised unless an attacker is in stealth, but you should be aware that the time it takes to get from the scan station to the flight controls could be critical.
The Lancers, Connies, 600is, Corsairs, Terrapin and Vanguards all sport M radar. They are all theoretically capable of the same exploration, save unless CIG starts modifying scan with antenna packages, etc. I have never heard that rumor. Given the smaller ships are faster, they should theoretically scan larger areas in the same time, so it is worth looking hard at the Lancer, Space Turtle and Vanguard. Of all these ships, the Vanguard is probably the safest to fly solo, since only a moron will attack a Vanguard. (bluster intentional) If you want your explorer to be able to haul away cool finds like alien artifacts and such, then the Lancer is your best choice, IMHO.
3) Do you think CIG will eventually buff or balance the 600i to be somewhat commensurate with it's cost (minus the BS "luxury" tax).
Doubtful. Its not a bad deal as is. Witness how popular it has been since day one. Some players fly them every day.
4) Will the Aquila be just as "solo able" as the 600i?
I don't think either of them is a good choice for solo exploration. If you're going to be alone, I would see greater benefit from the higher speed of the Lancer. Running away needs to be the plan for all soloists. If you fly a 600i alone you should expect to have players take it away from you often. Pirates will be looking for situations EXACTLY like the one you create, with good reason. A free 600i is a nice catch, and an Aquila is pretty good too.
5) Will the lack of a snub on the 600i preclude me from any exploration gameplay loops?
Yes. Neither the 600i nor the Aquila can pass through the smallest jump points. A snub might (can they open jump points? I don't think we know yet), but that would be a little foolish, to leave your ship behind unattended. This is another reason to fly a Lancer if you have to solo. Solo explorers should be looking for the smallest jump points, since these present a special opportunity. The Lancer is the largest exploration ship that can pass through a small jump point.
6) Will the 600i or Aquila be getting another design pass/major tweak/rework in the near future (probably a silly question for Alpha, but yeah...)
Probably not.