IE9 supports the three modes (7 8 and 9) that you need and 98% of the time they mimic the native versions of each mode pretty well. If you're still testing for IE6...well....stop
With regards to IEs developer tools - I didn't realize it had any (sarcasm). Up to IE9 they're absolutely useless so I simply don't use them. I do all my building and debugging in chrome and then check IE in compatibility modes to make sure I haven't left any trailing commas.