Hi Rob,
The enemy spawning is randomly. There are 5 lines where they could appear. Only in first level, active lines are 4, but in other all 5. Spawning is not just randomly actually, there is special coefficient, that increase spawning regarding time and enemy kills. So, in the end of level or after some time in Survival mode, the enemy spawning is much more frequent than in the beginning of level/survival.
The cloud system actually works almost like You said. When it is over cactus, You can see how the health bar is reducing and only after some seconds, defender disappears, because it took all the health. I could make quite easy that cloud system affects also gnomes and stones, but less than plants.
You have so much good ideas and I could expand and improve this game, but I think it is better to invest the time and these ideas in other games, which could probably be sold for money. What do You think? Or I could upgrade this game and sell it too?