Well, it's been a long time since I originally opened this thread - but today I finally had a holiday, and enough spare time to give this issue proper attention - with the invaluable help of my father. I definietly owe you guys a feedback, even if not a perfect one.
First, I rechecked the fuses and the SJB, to no effect. In my third attempt, I decided to open the center console and check the PRNDL light itself (erroneously indicated as an incandescent bulb on the electrical diagrams, but in fact an LED with a whiteish plastic cap), which proved to be working, according to the multimeter readings. So I tracked this LED's wires (black = negative; blue = positive) to the C3245 connector, unplugged and tested them there. There was no current coming to the LED. Using an adjustable external generator, I fed the circuit with very low current/voltage, and slowly build it up to 250 mAh/2V. Through the process, I saw lights starting to shine very dimly. At the final setting, most of the previously non-working lights on my Edge were at full brightness - except the Cruise Control keys, on the left side of the steering wheel, and the puddle lamps (being old bulbs, I suspect they may very well be blown, and already bought OEM replacements).
I finished the process by jump-wiring the positive blue and brown wires inside the dashboard, safeguarding this new circuit with a 4A fuse. Now certain lighting elements (the audio controls on the right side of the steering wheel, the headlamp switch and the Info/Setup/Reset buttons) are continuously on, when my car is on. Certainly a downturn but, to be honest, not one that bothers me nearly as much as having a pitch-black PRNDL on a dark parking lot.
The fact that only a couple of lights didn't turn on hints that the problem wasn't caused by a catastrophic failure (no huge short-circuits or widespread corrosion here), and that the cause is probably located somewhere in the "lower" parts of the lighting circuit, just after the SJB.
What I did qualifies, at most, as a simple quick fix and, though I tested it on all possible settings, I am not sure about its long-term effects. Plus, I've "fixed" only about 80% of the problem. Being so, I advise caution (and good luck) to anyone following my steps.
So far, I'd rate my issue as partially resolved. As soon as I get more free time, I'll check the remaining issues and post my findings here.
I'd like to thank the community, particularly enigma-2 (does your nickname has something to do with the German 90's New Wave musical project, by the way?), for the prompt support and precise responses to my previous postings. This forum is certainly THE source for everything about our beloved Edges!
JPDR's post in '09 Edge SEL dashboard/console lighting was marked as the answer