First of all - it looks like it haven't quite "clicked" when you closed the buckle, hence it was slowly working its way out until reaching the recessed part which got bent and at which point it fully released.
Second - unless it's cracked, there's really no reason why you can't simply hammer it back straight and use until they'll send you a new one. This is a fairly soft steel, so no worries. Just make sure it "clicks" when you close it. The recessed part isn't subjected to any heavy loads anyways, it's the long, straight neck that does all the work.
It clicked and I always double check if the mechanism is closed. I was riding more than an hour before it unclipped so I guess that it was no "user error".
I'm not sure about "hammering it back" as multiple bends could weaken the material so next time it could break. Maybie its OK to do it but I don't want the same scenario happening again.
The main problem is not the bended pin (as it was OK before it unclipped), but probably the locking mechanism which let the pin to unclip under heavy load. So no matter if I hammer the pin back if the mechanism will let the clip go again.
Well, for one thing you've been riding for a good while with the pin un-clipped. See, with enough tension on the spreader bar's hook, the pin would stay in even if it is not locked-in 100%. So you might be right - and there's a problem with the locking mechanism, but it's so simple that - given your limited options - it would be silly to simply discard it. Of course - the safest thing would be to replace the whole spreader bar, but for the time being you can just fix it up.
Also, bending a piece of metal in its original shape once doesn't quite qualify for "multiple bends". And like i've said - this is a fairly soft steel, so unless there are some visible cracks - straightening it should not greatly affect its mechanical properties.
At the end it's your call.