1st....I disagree about the needing to shoot in the f/16-f/22 range. In fact, I would strongly advise against that. f/13-f/16 is going to be the sharpest range and your shrapness will start to drop off above f/16.
Flash definitely helps....BUT, I don't see exactly what you are complaining about personally (this is a compliment..stay with me). I think the photos you posted are very good....much, much, better than I see from a lot of new macro shooters (better than I did my first week out with the Tamron 180. Don't sell the lens...stick with it and practice, practice, practice...it really does get easier the more you do it.
That said, a flash setup can give you worlds of flexibility in your shooting allowing you to shoot scenes (like in the dark shade) that you normally couldn't get. An SB-600 or even 400 through a $20 minisoftbox mounted to a $20 flash bracket (will position the flash to the side) and a $35 sync cord will give you awesome and dramatic off centered lighting. Rather than sell the lens...save for a flash and the things I listed above.....until then, practice your technique.
Technique is one thing people forget about...more important on macro than any other form of photograph...I'd venture to say more important than all other aspects combined. Tuck those elbows in and hold them tight to your sides. Hold the camera with a firm grip, but not overly tight. Set it at the mag ratio you want and rock back and forth for the focal plane firing at will as you pass through the focal plane in your viewfinder. When shootng....hold your breath. I always am out of breath after shooting a subject and it takes me about 10 seconds to catch my breath after a series of shots.