These two I found the most ridiculous. I'm not going to address the second, but for the first, assuming you're basing it on your unvoiced principle of doing everything like Epic did, it's total hypocrisy. Have you looked at Capture1, Capture2, Capture3, Capture4, Race2, or Race4? Yes, Epic usually made their levels 256x64 or 128x128, probably because there used to be a constraint against any other level sizes that they later removed in subsequent versions of level files, but they didn't hold to it as an unbreakable rule. In fact, many of their levels that are 256x64, you'll find, don't even use the whole space, and would have been better saved with a smaller width in order to reduce filesize and presumably memory use.

Actually, while I'm at it, let's point out some other ways in which Epic deviates from the ideal version of them that I think you've built up.

Nick and Dean made the tilesets; Cliffy and Jeh made the levels; Alex made the music. Those are different people. If you say that's okay because they worked as a team and so you want everyone to make levels as teams, you're just going to get fewer levels, and thus conflict with your point 16.

Castle; Labrat (foreground); Tubelectric. Again, not an exceptionless rule. Really, I think part of the issue here is your rule 1. There's plenty of reason to make a looping picture in a tileset that you're making for your purposes alone; we see this in its most extreme form in .j2t files that are nothing but text used for credits or introductions or what have you. But the more everything is a looping picture, the fewer options there realistically are for other people to use your tileset differently, and while I don't exactly have hard numbers on this, I would guess that one of the top contributing factors to how much a tileset gets used is how versatile it is. On the one extreme you have tilesets like Desolation, WTF, and to an extent Odyssey and 7th Lava Fall, which are extremely composable in nearly all their layers and thus lend themselves to a whole variety of images. On the other extreme you have 3D Battle Pack, although I should note that all those tilesets have been used for other levels at least once. If you're only trying to please yourself, there's no reason not to have a bunch of large images for the background; it does look good. But the more you want other people to use your tileset -- in conflict with your rule 1, which you must admit is scarcely a universal principle -- the more this point becomes hazardous to uphold. And while I'm waffling...

There's no right answer to this one either. I think lots of people would agree you that this is some sort of ideal, but it conflicts with three different practical realities:

• Most people use other people's tilesets. Either they may wish to use the tileset in a different or more complicated way than its author originally intended/expected, or the tileset simply wasn't built with this ideal in mind. Someone may want to use tiles in the background that are entirely different from the ones from the example level, e.g. many tilesets built using Damn or Jungle, since those sets don't have any layer 4 background tiles of their own. And again, interesting as a world would be where everyone made their own tilesets all the time -- perhaps what we are approaching in JJ1? -- that world would have a slower rate of production.
• Even if you make your own tileset, there's another ideal: make your tileset compatible with 1.23. This means that your tileset can be no larger than 1020 tiles, and it's very easy to run out of space with that number of tiles, especially if you want to allow room for more than three animations. Rarely will you see a tileset in which every single tile with transparency has an equivalent version over the layer 4 background tile, and even if you did...
• Once again, level makers are creative. Let's say someone is being very respectful of the tileset maker's intention and is using all tiles in the way they were intended to be used. The main wall type is the main wall type, the layer 4 background is the layer 4 background, and so on. But then suddenly they run into an issue: there's no tile that has a 22.5 degree sloped floor in front of the right edge of the layer 4 background! And there's no reason to expect there to be, either. Pretty much the only tilesets you'll see that even attempt to support that level of variability are Blade's (Forest, etc.), and even they can't cover every possible interaction because there's simply no room. The tileset creator can never be reasonably expected to provide for every possible interaction of wall and non-wall, unless maybe if it's a very simple tileset where everything is a 32x32 block.

Okay, so I feel kind of negative now. Here are some of your points I completely agree with:

Absolutely. We get some really weird "Evening" sets this way, but it's still much better than a quick brightness effect. It works best with 8-bit drawing styles, but those should be encouraged anyway...

While part of this is a matter of which tileset you use (looking at you, Haunted House), it's definitely a very worthwhile ideal to strive for. More layers makes for prettier levels! (And you left this ambiguous, and I would agree that it's a good ideal for both level and tileset creators.) To this I would add only that foreground layers are also nice, as long as they're either unobtrusive (Labrat, Diamondus, Tubelectric, Castle once they removed the giant pillar) or only appear in select parts of the level where visibility isn't at such a premium anyway.