1. @santosh, if you prefer bass that complements the fronts and simply supports them with the low-frequencies in such a way that the sub is invisible, 10" would be enough in a hatchback. Not to put ideas into you, but 2 x 10" would be better than a 12". And IMHO one 12" will be better than a 10" for most people. Overall a 12" will be better return on the money than a 10".
In a sedan, a 10" sub may not be able support to the fronts well enough. Not as much as a 12" sub will be able to. There will be a bit of loudness lost by the time the bass reaches the cabin, so a 12" will be definitely better in a sedan.
Even in a hatch, I'd recommend a 12" as you can always turn the bass down to the level you need. The bass from a 10" might be better for the kind of bass you prefer, but I'd play safe and get a 12". They don't cost that much extra, and you can always the dial the bass to be a bit tighter by adjusting the encloure volume (downwards) a bit.
Another thing (not that important, but for the sake of considering everything) is that later, if you want to sell the sub and upgrade, you'll find more buyers for a 10" than a 12".
SK456 runs a 10" IDQ in his Santro (last I heard, but you never know with the Delhi boys, heh heh) and maybe he can comment further.
2. Bass will travel into the cabin even without enough air-space to travel. But some opening between the cabin & the boot will definitely help get more defined bass into the cabin. The openings for the rear-speaker on the rear-parcel tray will be enough. You don't need to cut more openings.
3. The Comps you have chosen are very good, no doubt about that! Your preferred sub options are also very good. ID has a new range (CTX) that starts from 8.5K/9K. Maybe you can try listening to them when they are available (in March end I think). In JL's lower range, I've listened only to the JLW3 which was quite nice. Of ID, I've listened to all of the subs (except the new CTX) and I'd buy them without any doubt for myself (I actually have). The IDv3 is 14K now I think and is a great buy.
You can always wait for a couple of months extra (after you buy the current equipment), research well, and spend 14k to 15K for the sub if current budget is a constraint. Don't let impatience screw with the plan, or with the nice system you're building
