The next time it stalls and doesn't start, spray some starting fluid into the throttle body. If it fires up, you know you have a fuel delivery problem. If it doesn't fire, you probably have an ignition problem. This will help you determine which path to take to find the right repair.
The last three quarters of a turn of the spring is flat, and coiled to a smaller diameter than the body of the spring, so that it can sit snugly around the locating nub welded onto the frame, with the rubber isolator between them. With most of the last turn missing, the spring is effectively resting on a single point. The first picture shows its proper ...
Is it spring or the strut-mount assembly which is broken !!
The spring coil is placed with the rubber (and ball-bearing, if rear) kit on this mount. please confirm if the steel spring is broken or just this mount assembly?
However, if you don't change the broken coil, it would spoil shocks system on the other sides too.
That spring can rotate and rip out the tire - catastrophic loss when entering a bend could mean driver, occupants, other drivers, and/or bystanders could be singing with the angels...
Ground the car instantly, it is not safe.
The basic issue from a mechanical point of view is that the spring (not the shock!) is what supports the weight of that corner of the car. Clearly, with a piece missing, it is not doing that as it was designed to.
If the car "bounces" while being driven, the broken spring may end up further out of position and not carrying any load at all.
Personally I ...
My biggest concern with issues like this is causing additional damage since the spring is no longer being supported the way it was designed to be.
Also, the rear suspension is (obviously) compromised, handling is going to be affected.
My recommendation is to park it until it can be fixed.