menos I don't think we missed your point. I'm not suggesting that you change materials if you are really getting what you want but you are expressing the desire to open up the shadows and increase sharpness. Your chosen materials are simply bumping their physical limits. The fixes that are most effective are more exposure or more sensitive materials.
Yes, changing the dilution of your D76 can change the look slightly. Diluting the solution can reduce the solvent effect of fine grain developers so a change in perceived sharpness/detail can technically be gained. It is worth playing with.
With fixing it is normally either complete or not, there's no good middle ground. If it isn't complete, it can negatively effect your results, but that is a significant processing mistake in my world. I check my fixer before each run and typically it clears my test strip in 30-40 seconds but I still fix for 5-7 minutes to make darn sure there are no leftovers. If the clearing test takes longer than 45 seconds I toss that batch of fix out and start with fresh.
As to the effect of agitation changes, here is a link to an article that can give you a little insight into how agitation can be used. http://www.apug.org/forums/forum216/...-negative.html