I think explanations do exist for point 1 and 3. The DPRK has quite strong military production. It wouldn't surprise me if they think sending over military surplus they don't need right now (the US is tied up) in exchange for oil is worthwhile. As for the 30+ year old weapons, even old societ tanks, when well maintained could be used on the battlefield. I imagine that the DPRK of all countries would have the incentive to maintain their shells in good condition