Retina iMac not powering up after upgrades, one diagnostic LED

From the top:

I'm really nervous about the thermal paste replacement, mostly because I've never used one of those graphite pads before.

Graphite pads, rather than using paste, wouldn't cause an issue. You didn't place the pad between the pins and CPU, I assume.

Although I looked at the iFixit tutorial beforehand, one of the things I didn't realize was that removing the heat sink from the CPU would also remove it from the GPU and its 4 VRAM chips. These 5 chips had a thick thermal paste (like K5-PRO) on them. I didn't reapply any additional paste when I closed them back up. I think this is normally okay, but now I'm not so confident.

The heatsink is one unit, yes - so removing it from one removes it from all of them. This should not have caused an issue. You're right that not re-applying thermal paste shouldn't cause the computer to abjectly not boot. It may cause issues with heat dissipation, for example, later down the track, and I would advise reapplying thermal paste before you put the screen back on, but it wouldn't cause issues right now.

For a brief moment the CPU fell (was stuck to the heat sink), and landed on the aluminum chassis of the iMac from a height of maybe 15cm (6"). I cleaned both sides with alcohol, inspected it. Seemed fine. Aligned it and put it in the CPU socket and continued.

A concern, but not a likely one.

I was really careful with the power supply and made sure that all connectors were fully in place.

It's receiving power? It isn't the PSU. If it was the PSU, you'd have no power.

The RAM was the right type, pins aligned and all. But the second chip didn't quite fit in snuggly. I really felt like I had to jam it, and still, the bevel in the corner of the chip was not perfectly flush with the groove on the outline of the RAM socket. It was really close though.

Remove it, and try to boot. I would bet on it being a RAM issue.