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.
One of the advantages of star grounding is that you define the current paths for some signals. With a ground plane the current paths are up to the electrons and the voltage potential. You may end up causing a voltage dip or surge for one component on the board at a location where a higher surge current passes near the ground pin during, for instance, a relay activation.
It may not matter for your board, you haven't provided enough information to determine that, but the answer to your general question is:
No, a ground plane does not replace a star grounding system in all cases, and the problems a star grounding system can solve are not all resolved with a ground plane. In fact a ground plane may exacerbate certain types of problems that a star grounding system can solve.
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A lot depends on whether you are doing a commercial PCB to be manufactured and sold, or a few one-off boards for your own use. If the former, then a ground plan is more important to keep radiated emissions down to meet FCC (Part 15, unintentional radiators), CE and other regulations, although I've seen plenty of commercial products with one or two-sided boards and no-ground planes. (On the other hand, one of my clients just made a six-layer board with three signal layers, a Vcc layer, and two ground planes in a sandwich configuration.)
If you only have signals up to 400 kHz, then it sounds like you aren't using a microcontroller (or if you are, you are using an internal oscillator so there is no crystal). If you have prototyped your circuit on a wireless breadboard or the like, and it works okay, then it should work fine without a ground plane.
Even if you are doing a commercial product, if your device is battery powered, and it has no signals above 1.705 MHz (which is true in your case), then it is exempt from FCC rules.