It took about a billion years of photosynthesis on earth before all the ferrous iron dissolved in the oceans was oxidized and atmospheric oxygen concentration began to take off.

great questionprobably not poison it directly, but you'd lose a significant chunk to oxidation reactions before reaching any stable equilibrium. the surface is essentially a massive reactive sink. mars has a similar problem, the perchlorate in the soil would react badly with a lot of things we'd want to introduce. the optimistic read is that oxidation reactions release energy and eventually reach stability. the pessimistic read is the timescale is geological.

Terraforming is an exceptionally energetic endeavor. Even if you had the perfect combination of icy asteroids with just the right amount of water, nitrogen, oxigen etc. and the means to hurl them towards Mars, this kinetic event would be so energetic that it would take centuries to millennia before the surface would cool to habitable temperatures. it's not physically possible to do it ex in the span of a human lifetime.

Ar the scale terraforming entails, the crust reactions with the new atmosphere are closer to a rounding error.

Well, oxygen _is_ poison. It's eager to react (sometimes violently) with almost everything. It rusts and oxidates perfect shiny metals and silicon making everything an oxide!

if the moon will be settled it will be settled by AI embodied in some kind of (nano) robot or artificially created life.

Terraforming anything looks really expensive. Ask a finance guy to run numbers on terraforming places with gravity too weak to hold onto a useful atmosphere for any length of time*, and give you his opinion.

*say, Earth's moon