Water Heater Part 2: Voltage tester detects hot wires at thermostat but multimeter reads zero volts

Hello, c/plumbing, I'm back for another round of troubleshooting and need your help again, please.

Background: Yesterday my water heater stopped putting out hot water. Since I own my home, I decided to troubleshoot on my own before I call a professional plumber.

Ninja Edit 1 (additional background): Yesterday when we first noticed a problem, we were getting room temperature water, just warm enough to shower but still somewhat chilly, like the temp you get when you're about to run out of hot water. As of today, our "hot" water isn't even room temperature anymore, it's the same as our cold water (noticeably cooler than it was on Day 1).

(back to the original post)

What I've tried so far: With power turned off, I removed the cover panel and checked the reset button on the top thermostat, but it was not tripped (no click), so today I picked up a multimeter and tested resistance and continuity on both heating elements as demonstrated in this video. Both elements passed both tests - the resistance tests read between 12.7 and 12.9 ohms, and the continuity tests beeped. I also did a ground test, which read no resistance. So far, so good.

Where I'm stuck: At the electric panel I flipped the power back on for my water heater so I could do a voltage test at the thermostats and heating elements as demonstrated in this video. Even though my no-contact voltage tester detects hot wires (first photo), with the multimeter probes touching the topmost screws on the thermostat and/or the two screws for the heating element (second photo) my multimeter reads 0.0 volts in AC voltage mode (third photo). I've tested the multimeter on an electrical outlet and confirmed it reads 124 volts as expected (fourth photo), so I think the multimeter itself is functioning properly. I tried switching the probes, which made no difference (as expected b/c AC).

Any pointers on what I should try next? Since the multimeter doesn't read any voltage at the top thermostat, I'm not sure if this indicates an issue w/ the thermostat or if my testing process is flawed. Thank you!

Photos:

Ninja Edit 2 (more photos from my original testing):

I've also touched the multimeter probes to the screws on the heating element even though I originally only posted photos showing me testing the thermostat - here are the two additional photos that I should've included in the OP:

Edit 3 (terminal-to-ground tested each terminal individually + more photos):

I went back out there and re-checked each terminal individually (terminal to ground) for the top and bottom thermostats and heating elements. In all cases, readings came back between 124 and 126 vac.

I appear to have reached the limit for image uploads on a Lemmy post, but I also terminal-to-ground tested the other terminal for the top heating element and repeated these same tests for the bottom thermostat and heating element, and all of them show between 124 and 126 vac.

Where to go from here:

From these tests I believe I can conclude that all of the components (both thermostats and heating elements) are at least receiving power from the grid, although I am perplexed as to why when I connect both multimeter probes to both terminals at the same time, the multimeter reads 0 vac, which seems different from the second video that I linked above (here's that link again).

Given these latest test results and the fact that both heating elements passed earlier continuity tests w/ the power off and wires disconnected, any ideas what to check next? Is it time to just replace all four components and see if we get our hot water back?

Edit 3.1:

I'll look again for a rogue light switch and check the individual wires and if necessary the conduit cabling as suggested in the comments, then I'll report back. I do have an appointment that I need to scurry off to at the moment, but I'll get back to this in a few hours. Thank you!