2. Show Develop menu bar

Try to restart you Mac in Safe Mode by holding down the Shift key at startup.

Safe Mode booting is usually slow because macOS performs a cleanup and verification tasks. After the booting is complete and you're logged in, restart normally your Mac and check if the Show Develop menu in menu bar works.

Did you try to quit Safari, execute the Terminal command and relaunch Safari? Did the menu show (regardless the grayed checkbox)?

Another think I'd do is to try to delete Safari's preferences located in your user's Library (com.apple.Safari.plist).

Move the file somewhere out of the Library rather than delete it just to keep it as a backup. After relaunching Safari, the preferences file should be created from scratch automatically. Any Safari's customization will be lost of course.

Try to enable it manually via the following Terminal command:

defaults write com.apple.Safari IncludeDevelopMenu 1

The final number "1" is missing:

defaults write com.apple.Safari IncludeDevelopMenu 1