Can you explain how the absence of a mainstream alternative scientific theory bears on the strengths and weaknesses of the one mainstream theory?
Firstly, there is not an absence of a "mainstream" alternative theory, there is a total absence of any alternative theory.
Secondly, it doesn't directly say anything on its own about the strengths and weakness of that theory. However, if 100 years have passed (as in this case) and nobody has been able to postulate an alternative theory, and nobody has been able to provide any data points that weaken the theory, it is not unreasonable to assume said theory has a high probability of being the correct one. Please note, the theory has morphed slightly over time as new data and new technologies have emerged. Darwin didn't know about DNA, for example. The core hasn't changed.
Should students be taught critical thinking, absolutely, and Creationism could easily be used to help such education. It is in fact perfectly suited for that. One could use it to show that Creationism is not a theory, it isn't even a hypothesis. Creationism is simply a new way of formulating an age old superstition. As such one could sit down, blow the Creationism nonsense out of the scientific water so to speak, to show just how infantile the thought-patterns of its supporters are. I do believe that would cause some uproar though.
Another thing that could be useful would be Christianity as such. Many people believe the stories in the Old Testament for example, are historically accurate. You could use those to show how reason and rationality blows those notions out of the water and that people who actually believe in that nonsense are ignorant, intellectually impaired or lazy etc. Again, though, I think you'd have some politicians up in arms over that. If you taught how much rubbish superstitious people actually believe in and exactly how retarded that belief is, parents would pull their children out of school. I am certain that the laws proposed here were not intended for such activities.