Optimization for Area has higher Fmax than optimization for Speed?

1) Synthesis occurs before fitting, so there just isn't enough information. It does it's best, but it's just impossible to do.

2) Synthesis settings really control a number of other settings, and to be honest, there are probably hundreds of other levers being controlled behing the scenes. Most of them are not black-and-white on whether they help performance either. If you were to take all of these into account, it would be impossible to run through all combinations and determine what's "best".

For the most part, they work as described.

If you have the time, try a few different settings. The Tools -> Design Space Explorer can run sweeps of different settings to find different things.

For the most part, if you're just barely missing timing and can't do it with code changes or more manual things, then it may be worth trying, but otherwise it's probably not worth the time besides a few basic things.