|"I can't find actual voltages for the chipset (set +0.1v to 1.200v)|
and FSB (set +0.2v to 1.400v), nor the +5v rail."
What readings you see in the bios depends on what the bios designer decided you can see in the bios version - brand name system bioses often have a lot less in them that those of retail mboard manufacturer's - and on what sensors are built into the mboard (or cpu) that readings can be generated from.
I've never seen a bios for a mboard ( that has the monitoring sensors that generates current voltage readings in the bios - old mboards don't have that) that does not have a current reading for what is supposed to be +5v.
The +3.3v, +5v, and +12v readings directly indicate the voltages the power supply is putting out, unless the mboard is seriously damaged - I've seen only one case where a damaged mboard produced false readings in the bios regarding those voltages.
The other voltages are determined by the mboard itself. I know from experience that when you "overclock" the cpu core voltage, the cpu core voltage reported in the bios may not be the actual voltage the core is getting.
If your mboard is damaged, some things may work that require e.g., +5v, other things may not.
If the power supply is damaged, all the power supplies I've seen the insides of source all their power grounds, +5v, +3.3v, -12v, and +5vSB wires from the same physical source on the power supply's board, respectively, but more sophisticated power supplies have two or more places the +12v comes from on the board. So - if the power supply is e.g. not putting out +5v in the wring from it to something, there is no +5v being put out by the PS for anything; if the power supply is not putting out +12v in the wring from it to something, the other +12v source(s) location(s) may still be putting out +12v.
"The DVD drive is KOed while the system doesn't know it (smoke residue on the laser head?), so no bootable CD/DVD for me."
If the laser lens was merely dirty and nothing else was wrong, while the computer is running, you would be able to eject and retract the drive tray when you pressed the button on the front of it, the led on it would light up when the tray was retracted when a disk is in the tray for at least a short while (that merely requires the power connector be connected to it - the data cable doesn't need to be connected) , and you would be able to click on the drive in Windows when it has a disk it can read in it and it should attempt to read it.
You can easily clean the laser lens with a laser lens cleaning CD, however I don't know if that would clean off residue from the smoke from a fire - in that case you may need to open up the drive and use some solvent on it to clean it, along with a Q tip or similar or a tissue or a clean soft cloth.
If you don't have a laser lens cleaning CD, most places that sell CDs and DVDs have them, or even some "dollar" stores have them for a buck or two.
The optical drive cannot work if it isn't getting both +5v and +12v power. The floppy drive, USB, PS/2 ports, serial ports, parallel port, all require +5v power. The hard drive(s) require both +12v and +5v but I've seen cases where it/they still work if there is no +5v, at least on an AT computer it does / they do.
"I dug up my old (ca. 2000?) floppy drive, which I keep just for
bs like this, and was dismayed to find that I can't power the
damned thing because it has what looks like a 3-pin fan
header on it, for which I have no adapter."
Floppy drives usually use a 4 pin male connector, but it actually only needs 2 or 3 connections - a 3.5" floppy drive does not use +12v - it only uses+5v - and there two power grounds (black wires) in the power connector from the power supply - the drive only needs one of them. The 3.5" floppy drive could have 3 or 2 pins on it's male connector and still work fine - but I've never seen one that has only three or two pins.
The wiring coming from all power supplies has at least one smaller female power connector for floppy drives, if not two or more, but sometimes it is / they are in/on wiring from the PS that has been wrapped up because it wasn't previously being used.
The floppy drive male power connector may have fins on a plastic guide that restricts where the female power connector can be plugged in - the connector from the power supply can't be plugged into the wrong pins if it has that - but if it doesn't have guide fins, if you get the female connector misplaced on the pins, you can easily fry the floppy drive's board.
All or nearly all the 3 pin headers on your mboard are for 3 (or 4) wire fan female connectors - see your mboard manual - +12v, power ground, and a third connection for rpm input readings generated by a sensor on the fan. Some newer mboards have a similar 4 pin header for the cpu fan. A 3.5" floppy drives uses +5v, only.