Do they also perhaps look to have a bit more overall fog?
It's always a little tricky judging color film when it's still wet.
The negatives look underdeveloped, that much is clear. Given that the temperature wasn't way out of spec, developer activity must have been too low for a different reason.
I did mix the different parts before adding water. I took a beaker, poured part A, then B, then C, then water. Do you think that's where I went wrong?
So far, this is the only real mistake I see in your process.
As I poured part C into the A+B mix, the solution quickly turned milk white. It felt like pouring milk in coffee. As I stirred, the solution became clearer, but developed some weird blotches, similar to drops of oil floating on water. I used a magnetic stirrer to do the mixing, and after a little while the entire solution turned clear, with no blotches.
OK, so what likely happened here is that the developer, which comes as an acid, reacted with the carbonate from the part A concentrate. This is in principle normal behavior, but what also seems to have happened is that the color developer formed oily globules. I've seen this happen with CD3; in e.g. ECN2 developer it's a minor concern if powdered CD3 is added to the developer during mixing and for this reason I always dissolve the CD3 in some water before adding it to the mix. In the commercial C41 preparation you used, this pre-mixing was evidently already done, but apparently in the very concentrated 'developer' you made by erroneously mixing the concentrates together before adding water, conditions were created in which the developer formed aggregates. My best guess at this point is that those aggregates did not go back into solution. Effectively, this means you would have been running a C41 developer with a low amount of CD4 developing agent, which explains the underdevelopment quite well.
I'd suggest getting a new kit (or at least a C41 developer; your bleach and fix are still fine), mix as per the instructions and do another test run.
PS: your image are probably somewhat salvageable.