Road bike on tarmac. What should the optimal weight distribution between the front and the rear wheel be for optimal grip while cornering on a descent?
If the frame fits you properly, getting in the handlebar drops should give you the proper front/back weight distribution. This is of course a generalization, if you have your road bike set up with and extreme handle bar height then this might not be the case. For example, if you have a very high stack height (e.g., all your spacers under a high rise stem) with compact bars (i.e., the amount of drop is low) you may have really bend your elbows to get the trunk of your body low enough and enough weight on the front tire.
The geometry of the frame and fork will also affect this as different head tube angles and trail numbers (which affects wheel flop - how hard the bike turns in with weight on the front wheel) will have a different idealized weight distribution for aggressive riding.
Because a lot of this is rider and bike specific you will have to do some experimentation (i.e., empirical approach). Try taking the same set of corners at the same speed (something safe but still spirited) in different body positions. Too upright and you should notice the bike pushes through the corner. Too low of a body position and the bike will feel like turn in too hard and may feel somewhat uncontrollable or very slow to react to rider input changes. In the sweet spot and the steering should feel aggressive, yet still react well to input. You will feel the most confident at this weight distribution.
If you can also find corners that changes in their arc or have multiple apexes this can be a great way to find a setup that turns in well, but is still reactive to corrections (ideal for descending where you may have to make multiple corrections).
Finally, if you find yourself overshooting (pushing through a turn or a bad line going in), drop your inside shoulder towards the ground, this will add weight to the front wheel, which will interact with the wheel flop and turn the bike in harder. I used this trick many times in criterium races, but will only work if you are not at the edge of your tire traction.
Doesn't really matter - if you're fiddling with brakes or trying to adjust your front/back weight, then you're not thinking enough about following the best line through the corner.
The best cornering technique is to "enter the turn slow enough to not slide", with your outside foot on the pedal, in the lowest part of its arc.
Braking while turning is not a good idea - it generally means you've entered the corner too fast. Your best escape is to follow the widest line possible without crossing to the wrong/other side of the road.
Also, look forward - keep your eyes on where you want to end up (ie through the corner and down the road.)