Thank you for your time and trouble. I have wasted enough of that now.
Your approach is to avoid the device files changing. Mine is that it's a fact of life, Linux handles it elegantly and OMV doesn't.
I have also come to the realisation that OMV is like a black box: only very few people know what goes on inside. So, I have gone to #6.
I still think that OMV is a brilliant piece of software for certain users and their machines.
Thanks again and have a nice day.