Hi Lynne,
Well it looks like I was asking for a lot, and in the end I just managed to do a bit of searching around and found a quick hack you had posted for another user here:
https://vborg.vbsupport.ru/showthread.php?t=261657
With only a slight modification to match the newer code, it works just as expected. I made m prefixes float off to the right and just above the tag/has-attachment/subscribe icons.
I realize that this original request I had was a little too complex because I'd have an actual column to deal with. Even in some sections of my forum where I don't use prefixes, that'd be ugly. And I didn't want to get into any conditionals mess.
Although it isn't exactly what I wanted, it at least allows me to have a simpler implementation of the prefix being moved in the Forumdisplay.
As for the breadcrumb, I am happy with editing some of the PHP files to get rid of the rich text prefix from there.
Many thanks.