@de Silentio,
Aristodler, hello and great post, I really like talking about this kind of stuff so I apologize for the length of this post.
These are topics that are aided by physicists so I hope one will pick up and guide us. I say that because I know very little about physics, but perhaps enough to be of use.
It is important to remember that the units of measures we use to understand the physical world are just measures. By that I mean it is easy to forget that time is just a manmade unit of measure that represents changes (succession) in the physical. Whatever unit we use to mark and measure this succession is irrelevant because the units are manmade tools. A second's duration is only what we agree to determine it is.
Time is perceived as linear because of the one directional progress indicated in "past, present, and future". Einstein's theory of special relativity (Lorentz transformation) indicated time is not a constant but is actually altered as one approaches the speed of light. Einstein used the same example you did of time being as a river meandering across the universe speeding up and slowing down. Time is not constant although meandering and traveling backward are still linear. In the case of meandering it is not a straight line, and in the case of backward only indicates direction of travel along the path. Jumping from past to future without going through present is the only way I see of time being nonlinear, and even a wormhole would not do this because you would start at present and travel to future or past through the linear connection of the wormhole. So I say time is linear succession without necessarily being in one direction or straight.
The wormhole, by the way, is thought to be possible and consistent with laws of physics. The problem is at the entrance of the wormhole (event horizon), would require negative matter to open, witch has not been found yet, and quantum physics and Einstein's general theory of relativity don't jive. That is why Einstein and scientists today are working to find a unified field theory, a theory that can explain every property throughout the entire universe.
Light is an electro magnetic force. It is both particle and wave depending on how you slice it. Photons are particles but they don't behave like we think particles should, I mean they travel in waves. The speed of light is what Einstein used for constant in his general theory of relativity E=MC2. The C represents the constant speed of light in a vacuum (celeritas, Latin for swiftness). This C speed is used in calculations but light passing through atmosphere or a gravitational field is slower than light traveling in a vacuum. So we should not think of light as being constant in all situations.
In quantum mechanics there are states of matter that behave in ways that seem to not make sense. At certain levels/conditions our words and concepts (and math) are not sufficient in describing the relations and phenomena that occur in quantum mechanics. This is heady stuff and I don't have enough knowledge to really discuss it so I will leave it at that.
I think Hume's discussion about identity, unity, and succession is a useful way of analyzing our perception of time. Each instance of time is separate from the next and so is a separate perception. If you look at a river today and then look again tomorrow and say that is the same river you are not correct because the water has moved on and is not the same water as yesterday. We unite separate instances of time and perception into one identity by our imagination and the objects real contiguity. If you came to the river later and it had been modified into an aqueduct with concrete sides and bottom you would not think it was the same river because of its in contiguous appearance. On the other hand, a magician may show you a handkerchief and then using slight of hand show you another one that looks like it and you would say it was the same.
Anyway, I will stop rambling now and give someone else a chance