1) This assumption does not affect the example. If the well is already somewhat poisoned, making it more poisoned is still Bad.
2) The chemicals are definitely poisonous, but also have non-poisonous uses. 'The does makes the poison,' as Paracelsus put it. You still don't want them in your well water, typically.
3) Increased presence of poisonous chemicals in the well is literally what is presupposed by the example -- it's not an assumption, it's a premise.
4) This argument is anti-chemical-in-wellwater, it in no way touches on 'chemical stewards'. It's weird that you introduce this here -- effectively, you're modifying the analogy, and then pointing to the modified analogy and claiming it doesn't work.
5) See 4
6) See 4
7) This is a pretty reasonable assumption, given that there is no 'Ministry of AI'.
8) The fired ethicists may or may not have been good at their jobs, but no ethicists is probably worse than fallible ethicists.
You want to deflate the wallop this analogy packs, but I don't think you succeed.
EDIT @ 19:10PDT: added ', it feels like an intuition pump. ...'