Finally got a chance to pull the Mac mini apart to start investigating the connectors on the logic board.
First up, the fan connector. This looks to be a 6 pin molex connector. I haven't had a chance to figure out which line is which, but am hoping to scope it out soon. Speculation from some friends includes +/- 12V, speed sense, and maybe some temperature sensing for the different lines.
The power button is connected over a 6 pin FFC cable to an 8 pin board connector that apparently only actually uses 2 pins (source: "iMac G5 upgraded to iMac M1"). The thickness of the FFC cables is 0.21 mm. It looks like the LED and IR sensor FFC is the same 6 pin cable although I'm not certain if it's the same length. The board connector is also 8 pins, although it looks like the on-board connector is attached a little differently than on the power connector. I'll have to look the next time I get it apart.
The 20 pin PSU connector was identified by some people over at iFixit working on 12V conversions, and it is almost the right connector. The keying slots (?) aren't quite the same. The Molex connector keys are short and square while the OEM is long and skinny. The pins do line up, so I think if I trim the connector it will fit in alright, but I'd love to find a better match if anyone knows of one.