So here's the latest update!

After a break from the project, I've had some time to finalise all of the minor aspects of the PCB layout (connectors, spacing, physical dimensions, etc.) It took an entire day, but putting it off would have taken an entire month.

This is, without a doubt, the final high-level circuit diagram:

The final layout:

I did have some concern about the spacing between the "7mA Shock" section and the "safe" section, which is only separated by 2.5mm at the closest point. I was under the assumption that 7mA through the human hand to ground was merely painful, but it seems to start to encroach in the "cannot let go" territory. As a precaution, I've made a few circuit modifications to allow a sensing resistor configuration that will only allow a current of 3-4mA (max) through the human body should something go wrong (60kOhm inline resistance). However, for this to possibly occur, four things would have to happen:
-->The 2.5mm isolation is breached.
-->Earth comes loose within the unit.
-->The connected 3.5mm device also has an unearthed ground.
-->The person has conductive hands, etc.
Although unlikely, this is still concerning, especially if I were to give/sell a unit to someone else.

Regardless, back to snappys:

The idea behind the white silkscreen section at the top and the terminal blocks up there is that in this way an IEC quick connect panel mount connector can be used at the end of the case to make the PCB into a single unit (as originally intended). However, this also allows the PCB to be removed from the 3D printed case and used as a separate mains-->audio PCB as part of a bigger unit (with terminal block input/output). This is also the reason for the terminal block next to the headphone jack, which cannot be normally accessed whilst inside the 3D printed case.

To incorporate the finalised PCB into the old case, the 3D printed design needs to be enlarged by about 60%.
Below can be seen an enlarged version of the primary case extrusion:

However, since we're 3D printing and not actually extruding aluminium, the case body can vary in width along its length! The base of the case has a smaller opening to allow the existing IEC connector to retain compatibility:

The same size expansion has been applied to the front and back panels as well, though the face decorations/labels are yet to be determined:

Here's a quick picture from a failed print to preview the size compared with the old case:

I'm really feeling the project fatigue now. I've been dedicating mind-space to this project for months now and there are so many other things I want to do! Having a "properly" done PCB design and case is nice an all, but the gimmick wares off VERY quickly. This project should be called "An exercise in engineering design: tolerance and perseverance" or something along those lines. I just want to breadboard sparkly things...

Anyway, the next step now is to print all of the cases to a satisfactory standard and make an order for the PCBs/components. Ideally everything will fit and the PCB will work fantastically the first time it's powered on.

I'm sure this will be the case...