I need replacement heatsink hold-down pins (spring loaded plastic pins) for the CPU heatsink on my A1176. On the second...

Depending just how old a Mac you want to keep alive, NetBSD is probably a good option. It's been years, but I ran it on a TiBook and found most of the onboard devices to be reasonably well supported. You won't get a glossy, high-performance graphical desktop from a default NetBSD install -- you'll have to turn to pkgsrc for that. However, I'd generally warn against running anything but OS X on even an old mac if you want a pretty graphical desktop and decent performance; the OS X GUI is *much* lighter weight than KDE or GNOME...

I don't think the repair is terribly difficult. I do think the instructions are suboptimal in a number of ways, recommending disassembly of parts of the machine which do not actually have to be completely broken down to access the CPU or logic board reverse side. However, I do think you'll need a fair bit of luck. The heatsink retention pins are fragile and there are several components which have to be _bent_ out of the way to reassemble the logic board and fan/drive carrier into the chassis. These things are not designed to be bent more than necessary to put the machine together in the first place, so it's possible to do fatal damage even if you're perfectly careful. Nonetheless I've done a few of these with only one problem, failed heatsink pins on the second replacement CPU in the same machine. I definitely don't recommend doing that.