Electronic – Is it clever to put the AGND ground pour ONLY on the bottom layer and the DGND ground pour ONLY on top layer

At least for this capacitor you seem to be able to place it on the top layer. If you would place it there at the same coordinates you would shorten the distance between cap and IC pins by at least 80% (you also have to calculate the PCB's thickness). I would definitely try to do so. You can even move it a bit closer. Don't listen to Russell :-) when he says that it doesn't make a difference if you need the via anyway; it's the distance between cap and the \$V_{DD}/V_{SS}\$ pins that counts.
Also, depending on the CPLD's power needs the 10nF may be a little bit small, though this might be more of a problem for FPGAs than CPLDs. Depends both on the number of gates and the clock frequency. Still, when I use a 10nF cap I place a 1\$\mu\$F cap in parallel, with the 10nF the closest to the pins.
Daisy chaining your loads on a single power trace is not a good idea. Instead make the power supply's output a star point and connect your different devices on different traces, each with their own decoupling.

edit
Your third screenshot is definitely the best, decoupling-wise. (I would even let the traces go straight down.) I see no problem with the ground plane, nor with vias connecting to it. Just don't place the via between the cap and the CPLD pins. Distance caps-CPLD should be very short, if possible even shorter! :-)

edit 2
I didn't pay attention to the package first, but your fourth screenshot makes it obvious: your caps' packages are huge. I see Mark made a note about it as well, and I agree with him: switch to a smaller size. 0402 is pretty standard these days, and your PCB assembly shop may do 0201s as well. (AVX has 10nF X7R in 0201 package.) A smaller package will allow you to place the capacitor closer to the IC, yet still leave room for neighboring traces.

Further reading
Choosing MLC Capacitors For Bypass/Decoupling Applications. AVX document
Using Decoupling Capacitors. Cypress document

My guess is that it may be the cheap Crossplay supply, which is coupling (capacitive from the mains side) it's switching frequency onto the signal/PC/mains ground.
I would try using another (preferably better quality) supply of the same voltage rating (with same or better current rating) to see if things improve. Or you could try a ferrite bead clamped on the cable of the current supply if you have one handy.
If you have any means of isolating the Crossplay from the PC (e.g. little 1:1 audio transformer or similar between Crossplay/PC) then that should also do the trick if it is as suspected.

EDIT - guess 2 :-)

If it's not the Crossplay supply, then it's almost certainly the PC supply. In this case I think the easiest way to sort it is probably to buy the little isolator you link to, rather than trying to filter. These things are very handy to have lying around if you are into audio in any way.
In the meantime you could maybe try things like plugging the PC supply into the same socket as the Crossplay if it isn't already, or using shorter/thicker cables.
Audio noise problems can be quite difficult to diagnose, but IME (I have a small recording studio) an isolator will usually fix things, and often the easiest option.
One more thing - if your Crossplay happens to have a differential input (unlikely I guess) then use that.