When to use ground plane cutouts?
2) I highly recommend AGAINST cutting ground anywhere near high-speed signals.  Stray capacitance really doesn't have too much of an effect on digital electronics.  Usually stray capacitance kills you when it acts to create a parasitic filter at the input of an op amp.

In fact, it is highly recommended to run your high-speed signals directly overtop of an unbroken ground plane; this is called a "microstrip".  The reason is that high frequency current follows the path of least inductance.  With a ground plane, this path will be a mirror image of the signal trace.  This minimizes the size of the loop, which in turn minimizes radiated EMI.

A very striking example of this can be seen on Dr. Howard Johnson's web site.  See figures 8 and 9 for an example of high-frequency current taking the path of least inductance.    (in case you didn't know, Dr. Johnson is an authority on signal integrity, author of the much lauded "High-Speed Digital Design: A Handbook of Black Magic")

It's important to note that any cuts in the ground plane underneath one of these high-speed digital signals will increase the size of the loop because the return current must take a detour around your cutout, which leads to increased emissions as well.  You want a totally unbroken plane underneath all your digital signals.  It's also important to note that the power plane is also a reference plane just like the ground plane, and from a high-frequency perspective these two planes are connected via bypass capacitors, so you can consider a high-frequency return current to "jump" planes near the caps.

3) If you have a good ground plane, there's pretty much no reason to use a guard trace.  The exception would be the op amp I mentioned earlier, because you may have cut the ground plane underneath it.  But you still need to worry about the parasitic capacitance of a guard trace.  Once again, Dr. Johnson is here to help with pretty pictures.  

4.1) I believe that multiple small vias will have better inductance properties since they are in parallel, versus one large via taking up approximately the same amount of space.  Unfortunately I cannot remember what I read that led me to believe this.  I think it's because inductance of a via is linearly inversely proportional to radius, but the area of the via is quadratically directly proportional to the radius. (source: Dr. Johnson again)  Make the via radius 2x bigger, and it has half the inductance but takes up 4x as much area.