I am a practical shooter, not of the bullseye persuasion, but I've loaded and shot a whole lot of .45 ACP ammunition.
First, the "little milled extension coming off the back edge of barrel that the bolt face contacts" is called the barrel hood.
Please don't confuse it with Robin Hood, or even Little Red... Well, you get the idea.
Second, the .45 ACP case is supposed to headspace on the cartridge mouth, and it does, but if you make the cartridge quite a bit too short, it will still work properly. This is because the 1911 pistol features "controlled feed." That is, the cartridge being stripped out of the magazine slides up the slide's breechface and under the extractor claw long before it ever fully chambers. If the cartridge is too short, it will actually "headspace" on the extractor claw! The extractor will hold it in its proper position as you prepare to fire it.
Third, bullet-seating depth is a function of reliable feeding from the magazine, as well as cartridge length. As you have surmised, case length is more important, in terms of function. (Accuracy is another thing entirely.) But a cartridge loaded so that the bullet impinges upon the rifling at the barrel's throat may be too long to feed reliably, or, for that matter, to even enter the magazine at all. So overall cartridge length is also functionally quite important.
Fourth, if your loaded and taper-crimped cartridges drop down into the chamber of their own weight, when the barrel is held at less than vertical, that is absolutely correct. If the rearmost flat surface of the cartridge's head is right at, or ever so slightly below, the end of the barrel hood, that, too, is correct. Chamber length will vary, from barrel to barrel. The trim-to dimensions given are supposed to assure that the cartridges will fit any .45 ACP barrel. The difference between a trim-to length of 0.888" and one of 0.893" is only five thousandths of an inch, so either one will work, probably in any barrel. (I have never had to trim a .45 case, even after repeated reloadings, since they are grossly understressed, and do not stretch.)
Last, the .45 ACP cartridge is extremely forgiving. Even if shooting bullseye competition, case- and cartridge-length are not things to obsess over, as long as both are within the normally-accepted plus-and-minus limits. As long as your cases and bullets are consistent, and your powder is measured accurately, you will be OK. Consistency is the most important aspect of loading for accuracy.