My suggested PPIs were just round numbers, with no specific intent. The main thing was just to encourage experimenting with various PPIs so each shooter can decide for himself how relatively important it is to hit that frequently discussed magic number of 300 or thereabouts.
I did pick the 100 ppi knowing that that is roughly the minimum DPI recommended by some commercial services I've used. And in the case where someone is using a commercial service I think it is important to submit a print at that precise minimum, just to see what comes back, verses the higher res images.
The most important point you bring up is that a lot of this is very dependent on the specific output device, which argues further that we can never answer these questions in a general way. It just needs to be tested.
>> But when I went to scan the prints to see if I could use them as a sample in a discussion in another forum, I was floored to see that 100% crops showed the 300PPI print as containing more detail (which led to further experimentation).
That would not be at all unexpected. The scanner is acting like a magnifying glass, and depending on how you viewed the scanner output (100% pixels??) that drastically changes the effective COC. You should have seen the same additional detail with a close inspection with a very good and suitably high powered glass.
I've sent calibrated resolution test charts to commercial printers just to test their work . I had to use a good glass to find what I was looking for. And in that case I was interested in how much had to be spent in order to produce my own calibrated test charts, commercial charts being quite expensive. I was pleased to find that even dirt cheap prints can be pretty good in that regard. It's a work in process.
You also did not mention the distance you viewed the prints in the various lighting, and any optical aids you used? That is all about applying COC in a methodical way to determine how much res you need.