Images are highly saturated on export from Lightroom.

You’re welcome 😊

Please open the Import dialog, and post a screenshot of the Apply During Import panel.

Also go to Edit > Preferences > Presets, and post a screenshot of the panel.

So you didn’t do these edits in LrC, and this is how the image opened in Develop the first time?
If so, there must have been a preset applied on import.
Please post a screenshot of the History panel for that image. Only the Presets panel was showing in your previous screenshot, so collapse it, and display History instead.

As far as I can tell, the image in Develop looks exactly like in Camera Raw, and I assume this is what you want the image to look like.
The sliders in Camera Raw are “all over the place” because you have set LrC to automatically save metadata to files. Edits are saved as metadata, and Camera Raw is reading and applying the metadata, so the same edits as in LrC are applied.

The very dark image in LrC Library is caused by the embedded preview – the image was underexposed. The embedded preview should be replaced by LrC’s own preview when you start editing, but for some reason this hasn’t happened.
To update the preview manually, select the file in Library, then go to Library > Previews > Build Standard-Sized Previews.

To avoid this problem in the future, stop using the Embedded & Sidecar option under File Handling in the Import dialog. Use Standard instead, which will force LrC to build its own previews on import.

This is most likely a monitor color profile issue, not a Lightroom export problem. Try recalibrating or replacing your monitor ICC profile and make sure the JPEG is exported with the sRGB profile embedded.

I had overlooked that LrC is displaying the embedded preview, which may be the cause of the issue.
Please post a screenshot of the entire LrC window with the image open in Develop.
Make sure that the Histogram and Basic panel are visible and expanded on the right, and that the History panel is visible and expanded on the left.

The LrC histogram indicates a bright image, so I think Photoshop is displaying correctly, and LrC is displaying the image too dark.
Did you change the monitor profile with LrC and PS closed?
If not, restart them both.

Also, please post your system info from LrC as requested.

This is looks like a defective monitor profile.
As troubleshooting, and as a possible temporary fix, try setting it to sRGB.

With LrC, Photoshop and Photos closed, go to the Devices tab of the Color management dialog (the Advanced tab is not relevant here).
Add the sRGB profile, then set it as default.
If this fixes the issue, you should ideally calibrate your monitor with a hardware calibrator, which will also create and install a custom monitor profile that will be more accurate than sRGB.
(if you have a wide gamut monitor, use Adobe RGB instead of sRGB)

If this doesn’t fix the issue, please go to Help > System info in LrC, click the Copy button and paste in a new reply.