Just a word or two. I had a machinist give me a formula for a fantastic liquid to put on stuck bolts, nuts or what ever. Fifty fifty mix of transmission fluid and mineral spirits. He gave me a small article taken from somewhere and it showed drastic improvement over anything in a can.

Tornado , The Jack Daniels cocktail John has recommended has merit should you not have liquid wrench or any other form of penetrating oil . Ideally a torch would better suffice briefly heating the cap screw heads . Should you choose to use the penetrating oil route John,s or otherwise , ensure to hammer the head forcefully a few times prior and after applying the penetrating oil . I like to allow overnight , to enable the oil to travel . I would use a torque bar which will allow better feel and leverage if you have a good bench vice . Alternately a impact can be used but go slow to prevent snapping off the head . Ensure to 1st tighten then loosen the bolt slowly sequentially until very little resistance is felt to prevent stripping out the threads on removal . Once you have successfully extracted the bolt check for good thread and wire brush and use a coat of neverseize to the threads and replace and retorque. If the splines are worn you will indeed require new spindle shafts and mating hardware , there should very little radial movement , if so replace or the helicopter vibration will reoccurr ! lol. Keep us posted Bud ! P.S. You may have to file the head to allow proper socket to be hammered onto the damaged head . Otherwise we usually weld another nut on the existing nut in the shop . You may also consider a 18" pipe wrench for extraction tool in a pinch or drill out the broken bolt and use a bolt extractor tool .