Between fertiliser, light and CO2, there will always be one "weakest link" which limits the plant growth (unless all three are perfectly balanced, of course).
Raising the two that are not the weakest link won't help, because it is the other that is limiting growth.
For example, if you have too little light, adding CO2 or fertiliser won't help - because it is the light that is the limiting factor. Similarly, if you have too little CO2, adding light or fertiliser won't help. Etc.
In a cold-water tank, without very heavy lighting and quite heavy fertilisation, CO2 won't ever be the limiting factor. Only when you have fast-growing plants (mostly tropical), with a lot of lighting, and fertilisation, would CO2 be possibly helpful.
Of course it won't help the emersed plants either way.
To me, adding CO2 to the axolotl tank would be a slight risk (because you are introducing another unstable element to the tank, which is always a small risk), for no gain.
You can lower the light a bit, yes. I haven't ever noticed axolotls disliking the light, though some individuals might. You can try it and see how they respond. You have plenty of shadow spots in any case.
Surface movement is always good for the water; if you can get an air stone bubbling it will sliiiiightly help with oyxgen and CO2 levels. Most axolotls seem to enjoy them.
And lastly: indeed, general recommendations are 20 gallons for one axolotl, 30 gallons for two - at the very least.