Ozzie lost his leg to a Boggan
Not instantly of course but slowly after Ozzie first encountered one. The whole film is Professor Bomba telling somebody the story.
This would account for the somewhat disjointed storytelling, seeing as Bomba himself is more than a bit scatterbrained. Professor Bomba is slightly faster than ordinary humans.
When we see him from a non-Leafman perspective, he often seems to be moving and reacting and talking too fast. Also, this would explain how he discovered the Leafmen. If his speed differential is less than normal, he'd be a bit more able to see them naturally. Professor Bomba is a mad scientist and the world of the leaf men is a unmada field he's putting out.
He has the axioms of Apokalypsi, that he used to build those helmets of his, and his obsession resulted in the creation of the leaf men. The forest is still doomed.
We're told that the Queen Tara held the forces of decay at bay for the sake of balance. Now that the forces of decay have no heir, the balance is upset. Very likely all that new growth will suck up all the nutrients from the soil. Nutrients that would've been produced by the forces of decay. While not an immediate problem, it will eventually become an issue.- Except there's nothing in the film to suggest that another Boggan can't simply assume leadership of the surviving ones in Mandrake's absence. They've just had their numbers and ambition curtailed to the point where they can no longer threaten to cause too much decay.
Mandrake wanted to use the pod to bring his son back to life.
It is explicitly stated in the film that the pod will always choose a queen to rule the forest; however, Mandrake seemed to think that it would provide a male heir. This is because he assumed the pod would choose his son as a vessel for the forest's power. The fact that Dagda was dead wouldn't be a problem — as Mandrake says, if the pod blooms in darkness, it belongs to the boggans, and their domain is rot and death. Ronin and Nod's father killed Mandrake's family.
Mandrake mentions that Dagda took after his mother, but we never see her or hear about her again. Boggans are apparently born as larva, but bugs tend to breed by the dozens. Ronin and Nod's father infiltrated the boggan base specifically to take out the royal family. They managed to kill the mother and nearly all of the eggs, before Nod's father was killed and Ronin was driven away. That's why Mandrake hates Ronin so much — he's more than just head of the leafmen, he's the man who killed his family. Like with Nod's father and Tara, Ronin has known Finn since childhood.
The few interactions they have seem to give off the impression that they've been friends for years.Tara was a Leafwoman soldier before being crowned queen.
Nod's mother is alive.
There's no mention of her being dead (none in general), so it's a possibility.In the Epic universe, rot can happen in the normal way.
The reason the Boggans are bad is because they add extra rot on top of what happens naturally, thereby tipping the balance. Once the Boggans have no leader, they can't do their job, thus restoring the balance, because rot now happens the same way it does in our world (with bacteria, mould, etc). Mary Katherine is the old woman in the original book
The movie was loosely based on a book by William Joyce called The Leaf Men and the Brave Good Bugs, which was about a group of insects summoning the Leaf Men to save an old woman's garden from an evil Spider Queen. The old woman seems to know who the Leaf Men are, and mentions that her father told her about them, which raises the possibility that she is actually Mary Katherine many decades in the future.