You are being somewhat reasonable about this, ( thanks ) but there is more to the story.

What specifically did the tech do wrong to break the bolt and what could they have done to not break the ( non critical ) bolt? In rust belt areas, bolts constantly seize and or break during removal. If a shop was liable for every stuck bolt, they would be out of business.

Think of this, labor time guides are based on new cars without rust. An older rusty car can easily add 30% more time to the job. Should the shop be liable for the extra labor?

Some MB dealers won't work on a car that is over 10 years old, rust bolts are one reason.