I think you should be wary of assuming that John Ross died in 1857: the headstone that you mention also records John and Isabella's daughter Catherine, and
"Also her husband John Ross 5th Dragoon Guards
died 2nd November 1857 on his way out to the Indian Mutiny"
Of course it is possible that both men died on the same day, but I suspect that someone has made a mistake.
Maybe your best hope of finding any record would be a newspaper death notice.
If you are a member of a public libray in Devon, you can access the 19th Century British Newspaper Collection at home, using your library card number:
http://www.rootschat.com/links/0l27/Perhaps searching for marriage notices of his John Ross's children would help narrow the possible date of his death, as the fathers of the couple are often included along with whether they are "the late XXX".
Can't say for sure, but I think it unlikely that John's remains could have been returned to his homeland. A possible lengthy overland journey, as well as the sea voyage would make it impractical, and expensive.