After finishing this guide, it occurred to me to test out a little known feature in MacBooks...
The MacBook has sensors that detect motion. When the sensors detect motion that is violent enough to risk damaging the spinning hard drive (and it doesn't have to be much), the head gets temporarily disengaged from the disk so that the risk of damage decreases. If you ever pick up your laptop quickly and hear a click, that is what is happening.
After installing a hard drive where the computer only ever expected an optical drive, that safety feature is lost. Now when I pick up my MacBook when the drive is spinning, there is no click. I put an SSD in place of the old drive, so I can't confirm if it still works in the actual hard drive area of the computer... but the one in the optical drive area definitely no longer can protect itself from rapid movement.
Still a fantastic upgrade, but remember to treat the hard drive in the optical bay as slightly more prone to failure than before.
These screws actually use a #000 Phillips bit. The kit for this repair comes with everything you need *except* that. The difference between the two bits is night and day. I'll bet some—maybe even most—people can unscrew them with the included #00 bit, but mine were seated pretty firmly. With work, I got two of the three out. I nearly stripped the last one beyond use before I went out and bought a new bit to try. I'm not sure how to suggest adding that size bit to this guide, but I think that should maybe happen.
When getting back to this step for re-assembly using the dual HD housing, just know that the screws might not actually fit. I found that the heads of the screws were actually too big to fit flush against the housing. So I only used two of the three screws to re-mount. One of them is at a bit of an angle, too.
Overall, I am still happy with this whole setup.