You could use 5/level if you want level 1 to spawn an enemy every 5 seconds. At level 10 it would be every 0.5 seconds and at 20 it would be every 0.25 second. This works well if you want infinite levels where ultimately an enemy will spawn every frame. You could also tweak the formula further to be
Where factor could be 0.5 for a slower increase of the spawn rate or 2 for a faster increase.
Another approach you can use if you have a max level is to use lerp. It would look like this:
lerp(start_rate, end_rate, (level-1)/(num_levels-1))
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.
For the second question you could utilize something like this: