What is the electron density in a capacitor relative to source/ground?
So far I'm trying to figure things out by visualizing the electron flow and density in different scenarios.

I believe this is a tempting but often misleading way to learn; electrons themselves do lots of counter-intuitive things, and for most circuit analysis problems you're better off thinking simply in terms of &quot;charge&quot;.
It is also important to remember that &quot;ground&quot; is just a label and that every component has only &quot;local knowledge&quot;.
So, in the classical charge-and-voltage-fields model: the capacitor gets a charge difference such that the potential difference across its plates is 5V, and the top plate is the more positive. (This will correspond to more electrons on the negative plate).
When the switch is thrown, there is a loop allowing current to flow around from the top plate of the capacitor, through the resistor, and back to the bottom plate of the capacitor. It does so.
Looking from the ground, it is now connected to the top plate of the capacitor. The top plate is 5V more positive than the bottom plate, so we label the bottom plate -5V. This will decrease as it discharges.