The chemical intervention is only sufficient to get the excess of CO2 out of the atmosphere, which would get the surface pressure down to maybe 3 bar of mostly N2. From there, we'd still have to cool the atmosphere down far enough to support thermophilic autotrophic microbes in the new surface lakes. They would produce oxygen, but slowly, so there wouldn't be any appreciable free oxygen available for a long time.

The primary concern would be that Earth-gas would not be a good enough lifting gas in the CO2-depleted Venus atmosphere to keep the floating habitat at a cool enough altitude. Hydrogen-filled chambers would certainly work for that, but you would have to bring that hydrogen with you to Venus. By the time atmospheric oxygen becomes a risk, you would just land the dirigible and burn the hydrogen in a controlled fashion.