Fun level, and kudos for doing some innovative stuff. There are however a few small gaps in the logic for failing level 3 that you might want to take a look at. The game ended a few times at that stage when I in fact had the resources and positioning to keep going. Having taken a quick scan over the lua file for the level, if I've read the code right, it seems the issue boils down to not recognizing when the player has seedlings in transit to planet 8 when the farm area is destroyed.
The survival situation unaccounted for arises as follows. It's all the more frustrating that it fails the level because IMO you can't get to that point in the first place if you haven't played really well all the way so far, and to have just barely eked out survival yet still lose, kinda sucks
When the final attack wave of level 3 sweeps through, if you don't have enough seedlings to face down the enemy outright on planet 7, and thus can't stop him from taking the planet, the only remaining strategy is to retreat to planet 6 and build up your seedlings while you wait for 7 to fall. Then when 7 flips, attack it again as the enemy vacates it to move on to planet 6. If you were able to muster enough additional seedlings, you can retake 7 in time to move on to 8 before the area is destroyed. But as I mentioned, if your seedlings don't actually arrive before the wave timer ends, you lose.
I've also seen the level end prematurely if the enemy takes planet 3 but you have hopped past him to retake a higher numbered planet. It may not be possible to win if that happens, but I think the level should continue to play out.