Had the following reply from the lovely folk at Scotland's People today and still no dice! 😬
I have checked and like you cannot find a Death Registered in Scotland for Isabella.
I found her sister, Eliza's Death in 1942 and note that it was her brother John who registered her passing. I have also found John's Death, but nothing relating to Isabella.
I have checked all possible spellings of forename & surname, without success.
The fact that her date of death is shown on the grave, but she is not interred there may indicate that she died outside Scotland.
I have had a quick look at Ancestry, but again could find nothing relating to a Death of Isabella, although I did find something on a couple of trees indicating that she died in South Dakota. Are you certain she was unmarried at the time of her passing, as any Death would be registered under her married name.
I am sorry I can help no further, as based on the information supplied Isabella did not pass in Scotland.
Good luck with resolving the mystery.