When I am working with numbered lists, I have problems restarting numbering.
In some documents, I can restart numbering without any difficulty. In others,
the radio button is grayed out. Sometimes, I can restart numbering, but that
triggers indents, new font sizes, and other unwanted formatting changes.

I don't want to use complicated macros because other people who don't have
advanced skills will have to edit these documents in the future. I've tried
creating a custom style based on "no style" for my numbered list (my personal
preference) and resorted to manually typing numbers, creating additional
custom styles (e.g., Numbered list P1, Numbered list P2), and ever variation
I can think of.

I've started with brand new documents and edited older ones. There seems to
be no rhyme or reason behind the unselectability of a very important radio
button.
I usually get my documents from others, which complicates matters. Clearing
formatting and pasting the text in a blank document has had mixed results.