I don't have an exact answer for all of your 質問 but here are some rumors that I heard that might help.
1. Susan Strong might be Finn's mom,It's not known for sure what happened to Finn's dad. Possibly dead.
2. (This one is far fetched) Before Simon/Ice King アナと雪の女王 the world Finn was placed in a cabbage because, they resist radioactivity. Then after the Ice Age he unfroze in front of Margret. Whom then adopted Finn.
3. There is a secret tribe/community of regular people.