I have no advice on getting the nut off, and as I recall it was 12mm or 14mm - certainly not 8mm.
( they are standard right hand thread - lefty loosey, righty tighty - CCW to remove )
Once that is done, use a small puller to remove the arms.
When you remove the cover panel, make sure you also clean up all the old leaf debris.
I would go with Bosch, mainly because they look so much better.
If the plastic cover cap(s) are missing, you can get them from Rob at NeedsWings.