You cannot Edit an installed Library. Libraries are in what we call READ ONLY MEMORY. This is very similar to the Factory (Preset) data; they are in what is called READ ONLY MEMORY, too. The difference between the Factory data and an installed Library is one was installed by Yamaha, the other is installed by you.

What you must do to permanently EDIT A LIBRARY:
Transfer (using the "Import Library" function) from ROM to RANDOM ACCESS MEMORY (RAM). Now you can make your edits and "overwrite" the data. This is done in the User Bank (RAM). Think of RAM as the 'work area'.

Now once you have this data customized to your liking, which means changing the volumes or any other edits, including removing data you don't wish to install... you can:
1) SAVE a new LIBRARY FILE
2) Delete the original installed Library
3) Install your newly edited ROM LIBRARY - they now, again, become as impervious to change as the Factory ROM Presets.

You can think of a Library as an always available Bank of data.
And much like the Factory Presets, you can play them, you can change them, but if you want make a permanent change you must store the change to RAM (the User Bank).

I'm so sure you've read this, before. Except now that you have an experience interacting with it, like bumping into a wall... You will take to heart what is meant by ROM and RAM.

Please see the following article:
Montage USER and LIBRARY Files

If you have other questions, post back here.

Bless your heart, BM. You're right, you've told me this before, and in the back of my mind was an inkling that the problem was tied up with its being a Library file - but the detail escaped me. As you say, an hour of frustrated glowering and stabbing should have imprinted the lesson indelibly upon what poses as a brain between my ears ...

... thanks for your patience too - being an idiot, and being told I'm one, are two very different things!