First, does Lr incorporate ANY development settings into the DNG? I realize it creates a rendering so I can view it, but how much of that gets written into the DNG itself?
By default, NO. You can tell Lightroom to write the development settings to XMP, but the pixels in the file don't change, instead some text is written to the XMP section indicating your development settings.
Second, does Lr always write an Lr-generated preview into the DNG?
I don't think Lightroom's previews get written into the DNG file.
Third, when another application that converts RAW views that DNG, does it honor or otherwise use any of the default rendering instructions Lr may or may not have written into the DNG? Another way to put this is to ask this: if I were to take the ORF, and the DNG Lr converted from that ORF, would that pair of images look identical when viewed by another RAW converter (say the Mac OS, or Affinity Photo, or Aperture, or Capture One, or RawTherapee)? If the ORF and its DNG look different, does that mean the Adobe RAW conversion is somehow baked in, at least as a starting point?
Certain Adobe apps will make use of the editing information in the DNG, if such information is present (and by default, it is not present, as I said above). Non-Adobe apps can't use this information. So your question is moot.
Fourth, I have say the same ORF but I crop it before converting. The DNG seems to have that information in XMP metadata upon DNG creation or export. But when I view the DNG in say the Mac Finder that crop information is ignored. Is that metadata about adjustments typically ignored by every application but Adobe's?
Mac Finder cannot display the edits created by Lightroom and stored in the XMP portion of the DNG file.
LR converts as is and preserves any history. In your case there is no history but if you had used develop all settings would be available in the DNG plus an extra history step “From Metadata” which represents the conversion.
If Lr's previews don't get written into the DNG, what does it use for a preview? the same one included in the original RAW (I kinda doubt this since you have the option of specifying preview size in the conversion dialog)?
And it also appears then that the Lr editing info (like the crop) is the same XMP as would appear in an XMP sidecar to a RAW, and likewise not used by other applications (although I guess a few others use it).
The bottom line, then, is that an ORF RAW and a DNG converted from that RAW should appear exactly the same (the same as each other when opened together) in say Affinity Photo or Aperture, right? since either no editing info was written to the DNG, and they can't read or use that info, and since the non-Adobe application would just do their own rendering of either files raw RAW, if you will. Correct?
DNG's have their own previews. If you do further editing you can use the Library menu as an additional option:
Metadata >> Update DNG Preview & Metadata
1) Yes if you have added any adjustments to the original RAW file, which you are as the Default settings are applied. That is unless you have Zeroed everything out.
Those adjustment get included in the DNG file, IE Inside the DNG file. No Sidecar XMP file needed.
2) There is always a Preview section in a DNG file. Whether it is created by LR or was there from the start I don't recall.
3) Only if it is an Adobe product like Adobe Camera RAW. No other program can read the LR adjustments. Just like LR can't read adjustments made by other RAW processors. The way the programs make the adjustments and how they are written to either the actual file or to a Sidecar files is different.
4) The XMP files you are seeing is created by LR for the original RAW file at time of import. If at the time of import you selected Copy As DNG there would not be a XMP file created. All edits made to DNG files are written into the actual DNG file.