The monkeys that the rats "raped" to create the species probably migrated from Sumatra to Skull Island before it became its own landmass.
After the lawn mower scene, the remains of the zombies finally stop moving. But wasn't it shown earlier that even the organs of zombies can move on their own? And some of the body parts that remained, such as the arms, looked like they'd still be big enough that they could keep attacking....
I would have thought for certain that the WLWL chairwoman's husband would have become a zombie from ingesting the zombie virus-infected pus that Lionel's mother squirted in his custard bowl, but maybe we just didn't hear about it, or this is one case where it's only the bite that transmits the disease.
Also, I'm not entirely sure if it was the stimulant or what that caused Lionel's mother to balloon up into a giant bloated zombie monster.
She was the only one to be bitten directly, but all the zombies were given the stimulant. Presumably being patient 0 was the root of it, though the stimulant might have influenced it.