Oh my god. I just LOVE it when I make a level, and with a few tweaks it turns out far harder than I intended for it to. =)
In this case, all I had to do was up the release rate to 20, and drop the time limit to 3 minutes (the time limit change alone eliminates two backroutes, while still giving about 40 seconds to spare on the intended solution).
Bumped it up from a mid-Cheesy to a early-Gloomy level. =D
Also, to ensure players catch on to this modification to bomber mechanics, I've put a bombers-only level as the 4th level in the lowest difficulty.