IMHO: No pun intended, but you do not really have a leg to stand on. Your contract with United dictated a "no changes permitted" clause. You accetped this when purchasing the ticket. This is why buying travel insurance is very important on a 'no change' ticket -- this can be as little as US$ 99. United could've been a bit more sympathetic to your injury, but it is following the Star Alliance regulations. So, I just do not see any legal recourse.

Which airline are you ticketed EX-Bali? Try and reason with them. Bring all medical records and ask to speak with the 'Station Manager' at the airport.

Try to submit whatever costs you incurred for a new ticket to your own medical insurer. Keep every bit of documentation.

I hope that you are feeling better.