How to change the default which columns are shown in Finder, but *only* when I switch to list view

The only thing that I can think of off the top of my head would be to get a list of the items in the current folder, find the item with the longest name, and then resize the width per that value. That's really all that's happening with the optimal width double-click.

You can do it on a per folder basis. From the folders you care about - I would imagine this is just your users and root harddrive on your desktop. That'll cover most cases.

Open them, and hit either command-J, or View -> Show View Options.

If you're in Column mode, you'll see a check box to then alway show the folder in that mode.

If you always start finder from a couple of pre-defined places, eg. your desktop double-clicking your drive, this ought to cover your most frequent usage.