Security = separation from iCloud / Safari passwords

RobAW:

What would I lose in terms of user experience/functionality on Mac/iOS/iPadOS devices by having 1PWD as the single source of truth for passwords on my devices?

You would just avoid confusion, and you would not have to worry about thinking where you have stored some particular password. I think overall you would just improve your user experience but using just a single tool.

had assumed I could just keep using iCloud/Safari/keychain for lesser passwords.

You can certainly do this, it will work, but there is no need to use two tools that do a similar job. Also, you would only be able to use these passwords on Apple devices and in Safari, while you can use 1Password in multiple browsers and on other platforms too.

If I DO then go to 1PWD only - *exactly how should I set all my settings on my devices so that I get this right and iCloud/Safari/keychain is turned off entirely? (BTW - sorry to be slightly whiney, but this guidance might have been helpful to 1PWD users from the 1PWD team at the outset.)

Here are the instructions for this:

Set up AutoFill

If I DO NOT, how do I get my 1PWD passwords intoiCloud/Safari/keychain to let that be my primary password owner (while finding a local device only solution for my more secure items) - can I use MrC to achieve this?

I don't believe I have ever seen a case where the converter would be used to go back to Keychain, but an option is always that to export your data:

How to export data from 1Password

At that point, you can add your items to Keychain if you prefer, in whatever form or category you find more useful.