I'll try and explain one of the points you're still missing relating to the part I've highlighted in your quote, and please don't take this the wrong way again.
The braking marker you're suggesting to use (+10 to 15%) is only relevant for the car you are driving... and if you're in first place. It isn't relevant to the people racing at the back of the field, especially on the opening lap of a race. In every set of circumstances above you've only used two cars, but once you get a pack situation that all goes out the window.
In an ten car pack, for example, if the first placed car brakes just whithin the designated braking zone by only a small margin, let's say it's due to being an heavy car with excessive tyre wear, then the other nine cars will likely be braking outside of the imposed braking zone because they cannot brake as deep or they'll risk rear ending P1 later in the braking zone. The ninth and tenth placed cars will certainly be braking a long way short of the zone.
In this case even P2 has possibly braked outside the imposed braking zone, which means he's then open to being shunted by every car behind him because the system would wrongly assess him as a brake checker *to everyone behind him* because he braked outside the zone. Then if you extend the braking zone to include P2's legitimate braking marker, P3 get's a penalty for brake checking... and so on. For sure one or two cars may slip up the inside of P1 due to his predicament, but that again just moves the problem from P2 to further down the order.
Or, if you extend the braking zone to the point it takes in every conceivable scenario for all 20 cars in the field, the zone would then become so long and so far back from the corner that the front runners could deliberately brake at the very start of the zone, so early that anyone behind them couldn't possibly predict their actions. Again this would lead to complications but with different results. The car directly behind would rear end the car in front because he was clueless why the car in front jumped on the brakes where he did, and then he'd get a penalty just because the guy in front was deliberately being a dick by stopping short.... and every car behind them, if now outside the zone, could be considered as brake checkers *to everyone behind them*.
This is why I tried to explain (maybe poorly) that braking zones can't be just a line across the track, they have to be able to take all circumstances into account, and not just for the front runners but for everyone, or penalties will still be handed out unfairly. Then we'll be back to square one again with everyone complaining about the penalty system, but for different reasons.
IMO the first step in the right direction is what many others have said in this thread... equal penalties for everyone regardless of DR rank!!!!
* edited because sometimes I don't get things as clear as I would like the first time*