Question regarding BoP

Your restrictor is set to 20%, which means your engine gets less air intake and therefore produces less power.
100% means you still get some air, so it's only 100% of the restricting capability.

Pretty sure the practical result will widely vary with different cars and engines. What I've seen so far on sports and super cars 20% will net around 0.3 or 0.4 seconds per average lap (~2 mins). It's quite subtle.
Increasing the restrictor to around 50% feels more or less linear, values above might be more severe. Values above 70% suddenly make several seconds difference and also make the cars very little competative, but it's a matter of what you want to balance (different cars or different driver levels).

Hard to tell, depends on the car used. I'd guess 20kg mean more to a Formula 3 car, while 20% restrictor are worse for street legal cars.

 

It depends a lot on the type of engine (turbo, normal aspirated, high rev, low rev etc) and of course on the track used (power circuit like monza, or momentum circuit)
Ballast (kg) might not be much noticeable but keep in mind that worsens tyre wear too, so for long races can be an issue.
Restrictor on the other hand, might make the engine lose max power, but doesn't effect much mid/low revs, so for a big displacement big torque engine, you need a high % restrictor to have some effect... Still, as a rule (VERY GENERIC) of thumb, a 100% restrictor will make you lose at least 2secs/lap

 

It tries to simulate what happens in a real engine. It means that the loss of power gets more and more important as the revs raise because it restricts intake air.
So what happens is that at lower to mid revs, even with a restrictor, the air that goes in the engine is enough to give fuel and have the best possible outcome. But as revs go higher, the air going in, is restricted and becomes less than ideal (from the original engine configuration). This means that you must inject less fuel and as such you get less power.
There is no way to gain more power on lower revs with a restrictor in AC. In real life, the engineers might decide for a different ECU programming or maybe camshaft (if regulation permits it) that sacrifices top power (since it is restricted after all) for more low/mid power. But that is a specific design choice, not a result of solely the restrictor application.
A higher percentage means a smaller intake opening (a smaller restrictor).