So for example if you had 50 levels and you wanted the spawn rate to be 5 seconds on the level 1 and every tick on level 50 your forula would be lerp(5,0,(level-1)/49).
In general you may be able to use some kind of easing functions to do it too.
Edit:
For the second question you could utilize something like this:
http://www.scirra.com/forum/weighted-probabilities_topic48421.html