I don't really like using flash (I actually hate it) but it's a necessary evil with macro I'm afraid. I'm really keen on the new Mark IV, if ISO 6400 is clean enough, I'd shoot that, get rid of flash and still have decent DOF. You could try a couple of things, I'd recommend the LordV coca cola diffuser - it does work, it's a very nice, cheap diffuser.
http://photography-on-the.net/forum/...d.php?t=223485
some shooters swear by one of the twin flash units that Canon makes (can't remember the model) and they use half ping pong balls to diffuse the light. I'm not sure how they do it. I mostly shoot macro with the flash mounted atop of my hotshoe, with a sto-fen diffuser. Not great, but does OK. I do have a flash bracket and offshoe adaptor, but I found that it was causing me back pain (took 2 years to realise this lol).
Dave