You could weld a new end on it & try that, but I'll bet without drilling and sawing, it's not going to come out. Drill a one inch hole through it. Use a sawsall to cut it into 5 or 6 pie shaped pieces. Cut down to the threads, but don't go into the threads. Now you can use a drift pin and two pound ball pean hammer and break out the pieces.
But... I'd just run one return. While it's a nice idea to balance the flow for better heat distribution in the boiler, I'll bet in 99% of the cases, it's not truly balanced. Water takes the path of least resistance. Unless you put pressure gauges on both returns so you really know, you just don't know! Just my opinion, but it's not going to hurt a steel boiler to have a differential in temps from one side to the other or from the top to the bottom. A cast iron boiler might have regulations on temperature differential between top & bottom, but a steel boiler just isn't going to get hurt. I'd leave that one plugged, use the one that's working, and have no worries. If you're really worried you can add a by pass valve that makes some water spin in circles from supply to return. That'll keep the temps inside the boiler similar.