Whoever reads this know it’s just a theory.

First phenomenom

Mushroom war, I beileve the mushroom war was as a result of a nuclear bomb or bombs but if your reading this you already know that. My question is what were they fighting. Evidence of a war is seen in many episodes from wreckage to a soldgier in the background of a picture of Simon and Betty now ice king but never of what they were fighting or a battle (at least that I have seen). Now don't close this once I say this (you've done well so far) but I think it was aliens, aliens which I beleive were rainicorns. You may be thinking you were so doing well till that point but think about it, apart from the UFO's seen in the background of a picture with rainicorn's dad when rainicorn's parents came to visit they were wearing universal translators. Hopefully your thinking okay but why fight Rainicorns. Well we know they have a taste for humans and from Bob rainicorn saying he didn't think he'll get to see another human proves their interaction also a beware of UFO picture is seen on the mystery train. Also why would a creature who can fly by dancing on light need a UFO, your probably thinking why do we have cars but there were seen to be used in combat, in the episode her parents jake draws a dog shooting at a UFO. I also believe soy humans are zombies which will explain rainicorns bad dream about half dead creature attacking her probably a hunting trip gone wrong (zombie Finn had the same color of skin as the soy human hand on the table where finn and the rainicorn's ate). So if they met and fought why the war with dogs and why is the destruction of the world based on nuclear bombs. Maybe a freak accident or a kamakazi like attack which will explain rainicorns retreat to the crystal dimension only to return to a world of magic and anthropomorphic dogs (anthropomorphic meaning HUMANIOD. incentive maybe ).Back to the war i believe it happened in Europe, Germany to be precise as PB speaks German and Finn is caucasian with blonde hair and blue eyes. The buildings in Simon's dairy also appear to look European he also mentioned getting the crown in a European sounding country. Ooo contains people considered royalty an idea probably passed on from European lifestyle (being ruled my monarchs).

Second phenomenom

Human survival. Since beautopia we know Finn isn't the last one but how come their still alive. Here is my theory. After the great boom the sky was covered with smoke, no sun means immense cold that only Marceline and Ice king could survive, I this beleive is also the ice age and dinosaur extinction theory you konw meteor,smoke reptiles rely on heat so they die out lets move on. So surving humans try and leave through submarine and end up in Ooo's underground (as seen in susan strong) this could explain Finn's fear of sharks (as seen in King worm) and the ocean. Your probably thinking that was years ago but the fish people having gills proves they return to water regularly.

Third phenomenom

Candy kingdom. I believe this and every one in it was created by PB as she has been seen creating candy people and objects. The fact that PB and Marceline know each other and where once friends could mean they where both survivors or PB is like Finn and showed up a few years back. I also believe Ice king's particular obessesion of PB is due to her pink hair as Betty had Pink hair in the picture.