I have the same problem on a 2014 iMac running Mojave. Its 3TB fusion main drive filled up as I was downloading a 5 GB file. The write failed to complete and immediately the disk became read-only.
Now no command in disk utility or in single user or recovery mode can escape read-only mode, so no repair or reformat or OS reinstall is possible. I see no ...
AFAIR there never was an official way to access data on an iPhone/iPad from macOS through Finder. It may be possible with a jailbroken device, but that's for somebody else to provide an answer on that.
There are various other ways to transfer data between iOS and macOS though:
use iCloud drive on both and store documents there
AirDrop (which, if it works, ...
In the past I was able to use iOS devices as USB drives.
You appear to be mistaken. This has never been possible without third-party software and/or jailbreaking.
It was possible to use older iPods in "disk mode" as storage devices. However, this feature was never available for the iPod Touch, nor for any version of the iPhone.
Some iOS applications ...