Update on my Walgreens-in-California situation: I did get my test results (negative!) back this morning from a swab on Sunday evening. They told me at the time that the sample wouldn't even go out until Monday evening, so in my case, Walgreens took less than 24 hours in the sense that I would likely have had the same turnaround time if I had gotten a swab 24 hours later. As I mentioned upthread, I had already doubled-down with Clarity to be sure, and I'm not actually sure I'd take the risk with Walgreens, but this is my data point in case others are less risk-averse.

Also: like many others have mentioned, Walgreens backdated my collection time by about 14 hours, so that could potentially cause a problem with Hawaii's 72 hour clock.