Last things first
I freehand sharpen and I generally aim for the 15° or less range. I probably give a slight right hand bias only because with my handedness, and the fact that I do not switch hands to sharpen the left side, the right side gets a bit more grinding. I suppose I aim for a 50/50 bevel but don't sweat it if I am not quite there.
Steering is often cited as why you should retain the manufacturer's bevel but there are a few reasons to be a bit skeptical about how significant an impact bevel symmetry/asymmetry might have. First, the bevel is small relative to the grind (geometry of the blade above the bevel) which is probably more the business end of steerage in cutlery. Second, often asymmetry in the bevel is a function of the hand made process. While the maker likely has an ideal bevel geometry, a person is doing final sharpening and likely leaves their own innate biases in the bevel geometry. Resetting the bevel to your bias is not likely to upset the balance of the universe...by much
. Third, symmetry, like bevel angle, level of refinement, etc is not a one-size-fits-all proposition. I recommend you try to match the extant geometry to begin with, not because it is right, but just to gain appreciation of what it brings to the table, then in subsequent sharpening sessions alter the geometry in some way or another to learn how each dependent variable interacts with final cutting performance.
Which brings me to your first point, why a more acute bevel angle? Imagine a theoretical edge of two planes at microscopic level, in theory the line where they meet is perfectly straight with no blemishes. In the real world there are scratches from stones' grit, material defects, waves in the planes due to manufacturing or material defects, etc. Ignoring those, and assuming the perfect scenario, the more acute the angle the two planes meet, the "sharper" the edge. If the two planes meet at a 180° angle, then there is no edge, just a flat surface of material, if the planes meet at 90°, the edge will be more sharp but will still be limited as a cutting tool. So if you are at 18° degrees per side (DPS), then the two sides of the bevel are meeting at ~36° inclusive. Dropping to say 12° DPS will give you a narrower edge, a sharper edge, at ~24° inclusive. The reason not all knives start with incredibly narrow edge geometries is because the more acute the angle, the less material there is behind the edge supporting it. So a 12° DPS edge is more likely to chip or roll than a 20° DPS edge. So for each user there is a sweet spot where the edge is as acute as it can be while still retaining that edge to the satisfaction of the user under that user's normal operating conditions.
I know that was wordy, and I beg Ken a Jason to set me straight if I got anything wrong, but that is how I understand it. Hope that helps!