Questions

1.
The baseline of a font is the horizontal line at the bottom of most characters in a font.  The ascender height is how high a character can typically rise above the baseline and descender height is how far below the baseline a character typically can go. Ascending height can be based on a 'd' while descenders can go off of a 'p'

2.
With the given ratios, a 14 pt font would span (14 * 300  / 72) pixels = 58.3 pixels

3.
The mapping is set up in such a way that 1 unit on the coordinate system corresponds to 1 pixel. Since the fonts are drawn based on pixels after being converted from points by above formulas, it is convenient having the units equivalent to pixels to reduce the calculations needed.