Abe,
I purchased a Kimber SS Pro Carry about a year ago after going through what I am sure you are agonizing over right now. Let me see if I can answer some of your questions, and then give you my opinion:
1) Yes. 45 Colt, sometimes thought of as 45 Long Colt, is a rimmed cartridge and is longer in length. It is usually found in cowboy-action matches in revolvers, though some rifles use it too. I am not exactly sure what ACP stands for (Automatic Cartridge Pistol??), but it is a rimless cartridge that will feed through the autos.
2) Probably marketing hype.
3) I have been told that Springfield makes a good 1911, but I am somewhat biased towards my Kimber.
4) SS is a preference. It is said to last longer, but a well-blued gun, taken care of, will last longer than you or me. SS is a little more obvious when you try to carry, so most people opt for blued so it "hides" better. Both will show holster wear, just in different ways. My Kimber has the bead-blasted finish, so wear spots will show as slightly shiny spots and may not be that visible. Blued guns will show holster wear by having less or no bluing in the wear spots. Both can be touched up, but most will tell you to just leave them alone. Besides, it will take LOTS of in and out of the holster before you notice anything. Think duty gun for a police officer.
As I mentioned, I agonized over Kimber and Springfield (didn't want to spend the money for a Wilson or Les Baer). I had lots of input from different forums, but in the end, I had to come up with what I was comfortable with. I bought the Kimber and I have never looked back and never had a regret.
My Kimber was VERY accurate out of the box and flawless (and still is, a couple thousand rounds later). I have had absolutely no problems with it and the only things I have done is put smooth wood grips on it (came with checkered rubber ones) and put a mag well flare (I use it in IDPA, so speed reloads are important). I use Chip McCormick 8 round mags and they are also flawless. Everything about the gun shows the care and pride Kimber has about their guns. The trigger is short and crisp. Finish is top notch. If I am not mistaken, Kimber uses high quality Chip McCormick internal parts, which are already proven and sought after on the upgrade side of things. Basically, you get a very accurate, semi-custom gun at much lower than custom prices. I would highly recommend any of their standard 1911 style guns (I don't have any experience with the polymer frame ones).
As for Springfield, they make good guns too. I don't have much experience with them, but I simply like the Kimber better.
Hope I have helped some and not added confusion!