So, let me recap the situation for Elements 2018 with ACR 10.2. In Camera RAW Preferences you choose either "Save image settings in Sidecar ".xmp" files" or "Save image settings in Camera Raw database".
With sidecars enabled a sidecars is created for Nikon NEF RAW files, whether the file is Read-only or not. With the database enabled no sidecar is created.
With sidecars enabled a sidecar is created for DNG RAW files, but only if the file is Read-only. If the DNG file is not Read-only, no sidecar is created and I suspect the metadata is written into the DNG file, although it could possibly be written into the database; I can't figure out which.
In ACR 10.2 I can choose the Camera RAW Defaults setting which, as far as I can tell, shows me the unedited RAW file.
There are some drawbacks to using the database: Adobe Tech Support can't tell me where that database is located on my computer so I can't possibly back it up; someone there even told me it doesn't exist. The advantage is that you don't double your files in a given folder. I'm switching from using the database option since around 2000 to using the sidecar option from now on.