I have a cobra glass top of this vintage so this has my interest.

Did the bolt break or shear at the location where the thread ends and
solid shank starts?

Where the new bolts you used "grade 8"?

I would not use stainless bolts as they are softer and will shear much
easier than
a "grade 8" heat treated bolt.

By the way 5/16" is exactly the same size as 8mm, difference is only .
002, less
than a hair thickness. And I would trust the heat treatment of a grade
8 5/16" bolt over a class 12.9 metric bolt.

I think I would get bolts long enough to have a solid shank all the
way thru the structure causing the misaligned stress
you decribe and only enough thread on the back side to hold the washer
and nut.

I'll be checking mine if it ever quits raining.....................

Joe Simmers