4 protons (fuse to form) 1 ⁴He + 2 positrons + 2 neutrinos + energy

2.  The detailed steps involved in this overall reaction do depend on the mass of a star.  For low-mass stars (our Sun and lower mass), the standard proton-proton (pp) reaction you saw in chapter 7 is dominant (although the CNO cycle does contribute some).   In more massive stars, the core temperatures are higher and that favors the CNO cycle outlined in chapter 9.  The two different cycles have different temperature dependence - at lower temperature the pp-cycle wins, and at higher temperature the CNO cycle does. 

3.  Other nuclear reactions (that don't necessarily produce energy) also occur in the core of main sequence stars, and some examples of those reactions appear in Figure 9-9.   Think of these as by-product reactions with little effect on stellar evolution; these reactions do explain the distribution of element isotopes we observe in nature.  An example of one of those reactions is as follows:

            ¹²C + 1 proton (fuse to form) ¹³N,

and the ¹³N nucleus is radioactive, with a half-life of 10 minutes.  Therefore, all the ¹³N spontaneously converts to ¹³C rather rapidly.  This reaction has a relatively low probability of happening, so only a very small fraction of the ¹²C in the core will convert to ¹³C via proton capture and decay.  In nature, about 1% of all carbon is ¹³C, and 99% is ¹²C.   Many other reactions can and do also occur in the core as shown in Figure 9-9 during the main sequence portion of a star's life.

4.  The heavy-element fusion such as fusion of helium to form carbon does not occur at all in main sequence stars like our Sun; it does occur in stars after they run out of hydrogen in the core and leave the main sequence.  In the most massive stars, this reaction might occur to a limited extent late during main-sequence life, but the dominant fusion reaction is still that of hydrogen producing helium. 

5.  Since chapter 9 focuses on stellar evolution through the main sequence, it seems premature to introduce the fusion of helium to carbon, except to note that it does not occur for main sequence stars because the core temperature is too low.  After life on the main sequence, a sun-like star's core is depleted in hydrogen, the core contracts and heats, the region of hydrogen fusion moves outward (with a non-fusing core of helium), the star gets larger and cooler and more luminous, and it leaves the main sequence region of the H-R diagram.  Eventually, the core temperature does get to the 100 million Kelvins needed to fuse helium into carbon - the star then has a relatively short 2nd period of equilibrium while fusing helium, but it's properties are far different than when the star is on the main sequence.  Soon the helium core depletes and the star changes again and still more advanced nuclear fusion reactions might occur (depending on initial stellar mass), but that's really material best covered in unit 5, chapter 10.