Hmmm...you've got a bit of a problem there Martin, haven't you? Typical 400 series Volvo, the bit that should be the simplest fights back the hardest!
If it were me, I would (carefully!) take a small hacksaw and cut down the "shaft" of the rotor arm towards the engine. Obviously you will soon reach that backing plate, but by then you may be able to get a screwdriver in and literally shatter what's left of the rotor arm off.
I think there is a genuine backing plate on ebay right now -
https://www.ebay.co.uk/itm/New-Genui...wAAOSwc~dd1nsC - but if you wanted to be able to not worry about damaging the existing one then please see my PM...
I have found that aftermarket rotor arms really vary in size. Some are an easy fit, some just clearly aren't going to fit in a month of Sundays!
Good luck
David