The Chagossians are not by any meaningful standards indigenous. The land was uninhabited when George Washington was rebelling against the British. If the Chagossians are indigenous so are old stock white Americans.

And Mauritius have treated the Chagossians like dirt for decades, with no signs of that changing.

None of this is to deny the Chagossians were extremely ill treated by the British, but the idea that the Mauritanians have any interest in the welfare of the Chagossians is ridiculous.

I have some sympathy for your position, but I'll add that the prevailing moral opinion seems to be "whoever got there first is the rightful owner". Of course you have to allow for armchair ethnologists not being particularly good at distinguishing between similar groups and later revisionism.

A lot of Pacific islands territories have complicated histories like this (e.g. Hawaii, New Zealand), but the focus usually ends up on whatever bastards most recently took over from the previous bastards (relative levels of bastardy notwithstanding).