This is correct behavior. Columns are defined at the site collection level or list level so deleting a subsite will not delete them unless they were defined as list columns.
You need to go to site settings on the parent site collection --> Site columns and then delete them from there.
I can reproduce your issue in my environment. It seems a default behavior.
Since "All documents" view and "Show Toolbar" have conflict, and it causes the Toolbar type changed in the source of library.
As a workaround, you can edit the source document library page and edit the web part to switch the Toolbar type to "Full Toolbar".
To Create a Content Type using PnP PowerShell, you need to use the command like given below:
Add-PnPContentType -Name "Project Document" -Description "Use for Contoso projects" -Group "Contoso Content Types" -ParentContentType $ct
Check below article for syntax and list of parameters and their description:
For official documentation ...
Ideally, by following the steps you provided it should hide the field from list form.
You can also refer the steps from (If anything is missing): Hiding fields in a SharePoint list.
Note: If your column type is Yes/No then you will not able to hide the field in this way (By design).
Also, if you are using Classic experience then you can hide the fields ...
Per my test in my SharePoint online environment, I hide the column in the content type by following your steps.
Then it can hide the column in the forms in modern experience or classic experience.
To troubleshoot the issue, you could create a new column and hide the new column in the forms to check if the same issue will occur.
And then create a new list ...
You can also use the document set feature to implement your needs.
In addition, there's two similar situation here:
You are looking for Location Based Metadata tagging. Go into the library in question, go the library settings, Under General Settings you should see Column Default Value Settings. From here you can set the default values for your folders.