1- Accepting aircraft with a Legally deferred (MEL) item
Assuming it didn't affect something like passenger comfort, e.g. APU U/S: 40C+ in the cabin on the ground, the fact that it's an approved deferred item means that someone else has done the risk assessment, calculated the repair interval and incorporated it into a legal document. In the case of multiple concurrent defects, this process is unlikely to have been carried out.
Personally, I'd have a problem with justifying a decision not to take an aircraft with an allowable defect unless there were obvious additional factors in play. Yes, you normally lose some redundancy when you despatch under the MEL but you can still go flying "safely" according to the JAA/FAA, etc. It does mean if you get a further unserviceability, you sometimes have to consider early termination of the flight - but that could happen at any time on a "normal" flight, anyway.
When it comes to loss of redundancy, how far do you want to take it? Would you go with a pack locked out - MEL say yes? Would you go with a brake locked out - MEL says yes? Would you refuse to fly a 767 because "it hasn't got enough engines"?
In my company I've never come into conflict with the MEL, except under circumstances outlined above. I regard it as a "go" document rather than a "stop" one, to keep the operation running. Bigger brains than mine have spent long hours working out what's acceptable or not and in the simple, single defect case, who am I to argue with them?
2- Tracking/monitoring Captains that increase the Cleared Fuel after dispatcher finalizes his flt plan.
We have this - you can even go online and check your own figures and see how they compare with those of others. There isn't much pressure to take minimum fuel apart from "please take flight plan fuel unless you can think of a good reason to take more, in which case please do...". It doesn't affect the captain's authority on the day to load what he feels fit.