Hi Sherele
Very young mousebirds are quite vulnerable when just born but also amazingly resilient. Depending on how young your little bird is perhaps the strange twitching could just be the typical response of babies when about to be fed vis a vis an excited quivering and slight fluttering of the wings in excitement and also to show the parents they are hungary !!! as they normally are all the time ! Let's hope it is just that but the fact that it is not exhibiting these signs when asleep indicates a strong possibility that he/she is just showing anticipation of a yummy feed. The leg will heal in time so don't worry about that. Keep him /her nicely warm as you are doing, feed it often and assist with it's pooping if necessary and he/she should pull through. Then be prepared for a loving grateful little feathered companion that will display his\her love for you non stop 24 seven!!! They are amazing little things and highly intelligent. They say they are as intelligent as a 3yr old human and full of nonsense and very demanding of your attention !
Thinking of you and wishing you every success in raising this little thing to full maturity - the reward is immense.