Hi Spunky,
The subject is a bit complicated. Let me take the last part first. Light speed does not become degraded or 'tired.' It is the same as it has always been (much faster than currently measured) in between virtual particles. But the more virtual particles there are to absorb and then re-emit the light photons, the longer it will take that light to reach its destination.
About thermodynamics. That is not a good argument to use in a number of cases. First of all, thernodynamics itself has to do ONLY with a closed system and the heat distribution within it. Its name means 'the dynamics of heat.'
The argument you might want to use, but very carefully (I suggest studying about it a little first) is the more general tendency of the universe toward a higher state of entropy. That means things in general are becoming more and more DISorganized. However it is quite possible -- and it does happen -- for a temporary decrease in entropy to occur, meaning some area can get MORE organized. This, however, always happens at the expense of even greater disorganization someplace else. Thus, the net product is entropy. The evolutionist claim is that entropy has been decreasing continuously in life forms for about 4 billion years -- or that life foms have become more and more organized (from one-celled to multicelled to highly complex multicelled, etc.). THIS is what is denied by everything we know in physics, and biology, too.
There are 'moments' of spontaneously decreasing entropy from what we can see, but this is always temporary, and the general and net trend is always towards more entropy, or more disorganization. So don't use the 'thermodynamics' argument because it is essentially an ignorant one. If you want to deal with this area at all, talk about increasing or decreasing entropy. You are on firmer footing there.
Now, finally, about space being curved. That is a mathematical construct only. We do not see evidence of that in reality. In reality, what we have seen from Hubble and other telescopes is a surprisingly general 'flatness' to the universe.
Light speed itself depends on the 'thickness' of the medium through which is must pass. You can see this with a straw in a glass of water. It appears to be broken, or offset, at the surface of the water. This is due to light needing more time to get through the water than through the air. The only places in the universe where we see 'thicker' mediums are near large objects. And here, because of the presence of secondary virtual particles related to activity in the area of the large body, we see light slowing a bit and deflected a bit before it passes the large object.
Hope that helps a bit.
Tuppence