My experience background: I've made my own beer and wine, although not for years now (couldn't drink while pregnant, haven't got back into it since). I brew kombucha all the time at the moment, and my training and work background is biochemistry, which involved a lot of microbiology and fermenting of things.
My thoughts:
1. I wouldn't have left it in cast iron. Transfer to glass or food-grade plastic. The metal can leach into the stuff and do weird things (that's the technical term). Also, you can rust your cast iron cauldron, which is a pain to get clean and good again.
2. Leaving it on the bench should have been fine, but a usual 'room temperature' for these things is about 20-23 degrees, so if you're in a hot and muggy part of the world you might be getting things moving more quickly than the recipe anticipated.
3. Depends on what 'funky' means. If it smelled truly awful, like gag-worthy, and/or was a strange colour, best to tip it all down the drain. If it smelt kinda like vinegar or saukraut then you are probably ok.
4. Or wait and see and drink VERY cautiously the first time. It might be just fine and delicious. It might make you very ill.
I figure, the secondary boil would kill anything that had started growing, but there may be some off flavors stuck in there.
Yes, probably that is the worst that will happen. But if you'd had a massive bacterial growth before boiling, and if those bugs were toxic, then you could still get sick from the toxins. Very small chance, but still present (there are two types of food poisoning - infection from 'bad' bacteria, or illness from toxins from bad bacteria).
That's the excitement of brewing. I'm pretty sure though (from the other side of the world, and not having smelt the particular brand of 'funky' that you describe), that you'll be just fine. It might taste a bit metallic/iron-ish but should otherwise be ok.
The first time with a recipe is always an experiment. Next time will be better.