Sounds reasonable, I can definitely see how the chemicals in the melted plastic plant could mess with the water chemistry.
Good to hear about the filter, its just that the old one will have the needed necessary bacteria(what little you may have left)and removing it all together could make things worse.
Im not sure about Carbon-Ammonia Nuetralizing blend.....doesn't sound good to me, like a chemical that doesn't wok that the LFS'll talk ya into just to get some extra money off ya. You shouldn't add anything to the water save fore your standard water conditioner(I use this TetraAqua/AquaSafe stuff, comes in a yellow bottle. Dunno if its the best out there, but it definitely did wonders in my tanks)during water changes along with the new water(mix the correct dosage into the new water before putting it into the tank).
As for your Cichlids digging, totally normal. Cichlids dig around in the substrate to make caves to hide, for breeding, ext. No matter how many times I move the gravel around, my Firemouth always goes and digs herself new caves. I've given up moving it back by now
I also have a question for ya.....do you plan on upgrading your tank sometime in the future? Everything you have on your list will get very large and are usually recommended one per tank in a minimum of 50 or so gallons. And Oscars aren't only messy, but they grow fairly quickly to my understanding....
Not sure if you were aware, but just a heads up if your not.
If you wanted to keep them all together for the long term, you're gonna need a HUGE tank and even then I'd be wary about keeping them all in there together....