I undersand what you are saying, but you are forgetting that the "reality" of what you observe light years from the event is not possible. You're not able to be at a star 100 light years away to see how long it's rotation takes. So you're going to see it as taking a certain amount of time, and if you assume C has always been constant, then you'll record it as "reality." And this is what scientists do--because it's all they are ABLE to do. No one can say whether what they're seeing is actually slower than how it happened.
Your film example is a bit flawed, the initial speed of light would be the rate at which the film is recorded. The final speed of light at the observation point (earth) would be the rate at which the film is viewed. If the recorded speed is greater than the viewed speed, that's the definition of slow motion.
I don't know what you mean by not ruling out all light slowing down. My examples apply to all light slowing down. It doesn't matter whether light moves faster than events, we could replace light speed with something moving at a literal snails pace and demonstrate the same effect.
Imagine you are standing 10 feet from away from your friend.
Your friend has a bucket of snails that are trained to always move at some speed limit which could be constant or variable.
Imagine your friend starts a snail crawling toward you every minute.
Lets say it takes 10 minutes for a snail to reach you. i.e. initial speed limit is 1 foot/minute
If the speed limit is constant:
After 10 minutes the snails will start to arrive at you, one reaching you every minute.
Your observations (one snail arrival per minute) will match reality (one snail released per minute).
I'm intentionally using variables for this next part so you can replace them with any numbers you'd like to prove to yourself this works for any case.
If the speed limit starts at some initial speed limit C and decreases at a constant rate of -X every minute:
The distance equation for this is D=(initial velocity)*(time)+ .5 * (acceleration)(time)^2
D=distance snail has traveled
time=minutes since snail is released
initial velocity=speed limit at time of release
0 minutes: The first snail starts moving at the initial speed limit toward you. Speed limit is C
1 minute: The second snail is released. The first snail is (C-.5X) in front of second snail. Speed limit is now (C-X) feet/minute, both snails are traveling at the speed limit.
...skipping repetitive parts
10 minutes: The first snail is still (C-.5X) feet in front of second snail. speed limit has become (C-10X) feet/minute, both snails are traveling at the speed limit.
To mimic the apparent halt in any decrease of light speed on earth in the present day, lets say the snail speed limit bottoms out at (C-10X) feet/minute.
The first snail arrives after the 10 minute mark. The second snail now has (C-.5X) feet to cover before it reaches you. The second snail is traveling at (C-10X) feet/minute.
the second snail arrives (C-.5X)/(C-10X) minutes after the first snail.
Since .5X <=10X that means that (C-.5X) >= (C-10X). It will always be the case that the second snail arrives greater than 1 minute after the first snail.
Your observation (snail 2 arriving more than 1 minute after snail 1) does not match reality (snails released every minute). It appears to you as if your friend is releasing snails at a slower rate than he actually is. You can plug in any initial velocity to prove that it doesn't matter whether light/snails are moving faster than the events we are watching.
However, the decay of cobalt is not motion. But I would ask you what is the decay rate of cobalt-45?