What should I do with the stolen money that has multiplied dozens of times over ten years, given my inability to return the principal to its owner and my ignorance of his whereabouts, and will I be rewarded for my effort in developing it?

A distinction must be made between stolen capital and the profit generated from it. The stolen capital must be returned to its owner or their heirs. If they cannot be reached, it should be given as charity on their behalf.

As for the profit generated from investing stolen money, jurists have differed on it, holding two views:

First: The profit follows the capital and is returned to its owner. This is the view of Abu Hanifa, Ahmad, and Ibn Hazm, and it is the more preponderant opinion.

Second: The profit follows the effort expended and belongs to the one who invested it. This is the view of the Malikis and Shafi’is.

The more preponderant view is the first, because the oppressor should not benefit from their oppression. You must dispose of this money and its profits and search for its owner or their heirs.