Swingarm cracked...

[Mr Gazza]

Another bullet dodged! That could have been very messy.

Do you know what the braced arm came from? Did you add the brace?
It begs the question; was the brace anything to do with the extra stress at the pivot? Probably not.
Those housings are properly cracked and yet appear beefier than standard. Could it be a well raced 851/888 arm?

I wouldn't bother with the bracing if it were mine, just extra unsprung weight for un-required stiffening on a road bike.
Your spare arm looks as if it may have come from an M900Sie, it has two holes on the rear brace which would mount the extra little mudguard piece of an Sie. But for all I know they probably all have those holes?

Your brake torque arm bracket is on upside down btw.