Hello
I swap the servicing of my Multipla between two garages.
I recall one telling me the horror story of how they couldn't get the rear arm bolt out and had to cut it out.
I've just had my Multipla MOT'd with the other garage, and it needed a new arm. They also had to cut the bolt out.
So whilst I'm not being very helpful with your case, take some comfort in knowing that you're not alone.
It took my garage about an hour extra to cut the bolt out. And the bolt from Fiat was about £15.
Thanks