I am not familiar with Bangkok and the the various districts around the city and so I would be grateful to know if the predictions for Bangkok over the weekend 15 -17 Oct are correct, would I have difficulty in getting from Mo Chit bus station to Don Muang airport by taxi. Anyone with relevant knowledge of the geography of that particular area?

A taxi should take you to the OLD airport Don Muang within 30 to max 45 min for approx 100-150 THB...

If there's any flooding at street level, the taxi will be able to take the toll road. So you should be fine.