If the user doesn't have save access in the application, you can't override that read-only restriction for specific fields or actions like changing status, adding attachments, etc. There are some odd exceptions in the list tab due to how IBM builds those dialogs where some features will work without it and others will not.
What I suggest for customers with this scenario where you want people to mostly have read-only, but write to a subset of the fields, attachments, etc. is to create a cloned application where you set the inputmode for the sections to read-only. Then even though you have granted save access to the user, all the fields except what you want them to modify is read-only so they're restricted to adjusting the things you want them to modify.