considering most Minecraft mods directly mess with the Minecraft code as opposed to going through a well defined API (forge and fabric only provide so much) you can’t really make that work without outright stealing Minecraft code
the best you can make would be resourcepack and datapack compatibility. maybe whatever molang stuff bedrock’s up to
nah, there are plenty of truth wannabes (freeze peach bigotry safe havens) that actively federate. just look at literally any competent server’s blocked instances list and you’ll see a few examples. there’s a reason why nobody* runs completely open federation
*: aside from people who’re friendly with that crowd ofc