Yes, there's certainly more work to do here.
And finally, I have something of some significance to contribute to this thread. I checked all of Flurry and found only level 8 to be impossible. In completing level 10, I found a neat trick for building across gaps. If you've ever tried to make a 7-builder stack with a high release rate (like in ccexplore's one-minute solution to Taxing 24), you would have noticed that all but two lemmings stop building and turn back. Well, this works over distances too - rather than building on top of the first brick of the previous builder, you can start on top of a later one - as long as it has just been placed when the next lemming gets there. Often this will require a change of release rate, but in this case the default rate works nicely.
The end result is a solution that loses only two lemmings - and could possibly be improved to 1. Replay attached (as you'd expect).
Also attached is the replay for Flurry 16. It's not that difficult, but not straightforward either. And 13 isn't obvious, but it definitely can be done and isn't worth uploading a replay for IMHO.