Wow, first I've heard of this. Pretty surprised as the quality of the 15/22's I've seen have been great but I suppose looks can be deceiving as to the inner workings of a rifle. I hope they can resolve the QC issues as there are a lot of 15/22 shooters our there.
In thinking about this for the last 2 minutes, it's possible that the ammunition was part of the cause of the runaway. If said ammunition was also dirty (as we all know that 22lr is) or lack of maintenance in general, it's possible that fouling blocked the firing pin from fully retracting into the bolt.
If the ammo being used is weak and causes the bolt to short stroke just enough to chamber a new round, but not enough to reset the trigger, the trigger could follow the bolt home without engaging the sear.
I'm not certain that I am explaining it as well as I would like because I'm feeling under the weather this evening and not my usual perky self, but that is at least one thing that should be investigated. And without having the actual gun in hand in the condition that it happened in, will probably never be fully resolved.
As for the incident of an out of battery, could've been caused by a squib that allowed the bullet to partially chamber, and we know that a rimfire doesn't even need a firing pin to go off, so just the action of the bolt closing on the partially chambered round could have been enough to light it off.
I'm just speculating, but this could have happened in my mind with ANY twenty-two just by the nature of how they operate.
I did see reference to some Remingron Thunderbolf ammo. That is the dirtiest ammo I have ever shot and will now avoid it like the plague due to the feed/extraction issues it has caused me in the past. Its one of the only rounds to give me issues in my 10/22 and Model 60, both of which usually will eat anything.