What is the meaning of a deceased's body not decomposing fifteen years after their death, and is it permissible to bury a new deceased next to an old one in the same grave?

The earth does not consume the bodies of prophets, due to the saying of the Prophet, may Allah’s prayers and peace be upon him: "Indeed, Allah, the Almighty and Majestic, has forbidden the earth from consuming the bodies of the prophets, peace be upon them."

As for those other than prophets, this has not been established concerning them. If the body of one of them does not decompose, this can be considered a karamah (a miraculous favor), but it remains among the matters of the unseen that cannot be asserted definitively except with evidence.

It is not permissible to bury two or more persons in one grave except out of necessity.