Lower ceiling = shorter speaker stands?

If the postion of your ears are "in line" with the tweeters of the speakers, there is no need to change anyting,I think. Reflections from the floor and ceiling are mostly unwanted, and as the reflections from the floor are easier to damp (but not eliminate) with a carpet, the reflection from the ceiling isn´t within the time window , where the brain is most likely to be confused by the reflected sound.

I don't think so... I think it would be more important to keep the relative positions of the drivers to your ears the same, regardless of the fact that the ceiling is lower.

Maybe you need a shorter chair?

Is there an ideal ceiling height?