In this case, the symptoms don't seem to clearly point in a single direction yet Paul - I'm not predicting a leak between the crankcase halves, which would be pretty unlikely unless it had been dismantled. However the cylinder can come loose, and needs to be checked. I've found your symptoms a bit mystifying, so far. Were the chain and bar fitted when it did its strange kick-back that sheared the key? I suggest you leave them off until we achieve some certainty that it runs properly without them. My experience with tiny 2 strokes using diaphragm carburetors is that they are tricky enough to begin with, without adding in extraneous factors like a load that may or may not be applied at any given moment. Essentially, your mystery crash-and-burn could have been just the chain seizing in its track for a moment, which would be unsurprising when we don't know how good the chain lubrication is, and we know the bar was bent.
I'm not yet sure that the fuel tank is behaving properly. However, chainsaw designs vary: some of them pressurise the fuel tank rather than using a fuel pump, or to assist a fuel pump. Can we get access to at least an owner manual?
Let's see if we can sort out the bare engine first, and then worry about the bar and chain.
