when mormns use the word exaltation in reference to the destiny of human beings, does it mean reaching a level of progress where the married couple has their own world to rule over?
I believe so.
do mormons believe that children who die before getting married can never reach a level of progression as high as that a married couple can reach?
If an individual does not have the opportunity to marry in this life, they would have the opportunity at some point.
since heavenly progression goes on forever, why would those in the telestial or terrestial levels never progress to the next higher level?
The concept of not progressing is in relation to the other Kingdoms. The rate of progression is exponentially different from Kingdom to Kingdom. An individual in the Telestial Kingdom will never be at the same point in their progression as anyone in the Terestrial, and neither of the two would ever be at the same point as someone in the Celestial.
what is it that prevents a person from transitioning from the terrestrial to the telestial and then to the celestial?
Telestial is the lowest, you have the Telestial and Terestrial reversed. See my above answer with regards to progression. It simply takes more time to achieve such. What is it that prevents most high school drop-outs from achieving the same career and financial level as most high school graduates? What is it that prevents most high school graudates from achieving the career and financial level as most college graduates? Or perhaps better would be simply the transition from poor to lower class or lower class to middle, etc. I do not know that a faster transition never occurs, but if it does, such would be extraordinarily rare.