My main question is this: because almost all my images are JPEGs (not RAW), are these XMP files actually used during LR import?
LR only pays attention to .xmp sidecars for raws -- it ignores them for every other file type. You can use the free Exiftool to see if Aperture is putting information into the .xmp sidecars that's not in the .jpgs themselves. If it is, you can also use Exiftool to copy the information from the .xmp into the .jpg. The documentation has examples that show how to do that.
I can't answer your other questions -- it may be that the answers may obviate the need to learn the difficult Exiftool.
Thanks, John. I experimented with exporting images out of Apple Photos. Some I exported with the option to also export XMPs. And I found, upon import into Lightroom, that there was no difference in the Metadata between those with and those without accompanying XMP files. These were all JPEGs so I assume your answer is correct.
My other question answered itself. Because Lightroom can manage (move, rename, delete) images in a source folder (like the Master folder within Apple Photos), then it's best not to mess with it and instead create a dedicated "Import-Lightroom" folder that always stores images from my Mac and iOS devices.
Some I exported with the option to also export XMPs. And I found, upon import into Lightroom, that there was no difference in the Metadata between those with and those without accompanying XMP files.
Right, that confirms that LR ignores .xmp files. You'll still need to determine if there is information in them that you want imported into LR. The .xmp files are just text files -- you can open them in a text editor and see if there are fields that aren't showing up in LR's Metadata panel (with the EXIF or IPTC presets).
I did a spot check in TextEdit and the XMPs mostly had Apple's face ID notations. That doesn't matter at this point as I plan to run Lightroom's People function anyway and start over and do it more thoroughly this time. Thanks, again!