Thanks for sharing the update. I can definitely help on this. Considering the line o research that says Sarah might have died in Cawnpore but the regiment had moved out in 1856, it is quite possible that she was buried with in the Kutcherry Cemetery or at Christ Church.
So what I can immediately do over the weekend is check out these two places and consult the caretaker and Father at the respective places and check for other pointers. To look for her in the All Soul's does not sound motivating at the moment.
Simultaneously, I will look out for John Mason in Meerut and the records in the orphanage in Agra. Whatever headway I make, I will keep you posted.
I would request you to be patient because sometimes there is a period of information draught and them wham.. it shows up. I know it because I have been through it. :)
Nevertheless, my support to you remains. Should you get any updates meanwhile, request you to share it.