In answer to your last question - I would say that creationism is testable in much the same way that evolution is. You can’t reduplicate it but you can look at the fossil record and the complexity of life and see if it is consistant with the view or not. Intelligent Design is simply a theory that much of the complexity of the genetic information contained in the DNA could not have been developed over time by random mutation + natural selection. So, it could be disproved by expiraments which show beneficial new proteins devoloping over time by random mutation + natural selection. Probably the easiest way to do these expiraments would be with Bacteria which have really short lifespans.
Due to my weaker background for macroevolution, I believe it is harder to test it in the lab. (There may be good examples of reproducible experiments, but I am not familiar with them.) It is, however, easy to observe its effects. I've previously mentioned Darwin's finches, plant reproduction problems, and geographic isolation.
Creationism could be true. The problem is that it is difficult to test its propositions. We can observe a lot of things that seem to be the products of design, for example, there's a mantis in Indonesia that looks exactly like a flower. But, it's not scientific to attribute the result to a higher power. Doing so invokes the supernatural, which has no place in science. What's more, claiming that "God did it" is tantamount to quitting, in my opinion.
When asked to reconcile the not-perfectly-circular orbits of the planets, Newton claimed that it was the work of God. It took another scientist, Kepler, to show that the orbits are elliptical.
We may never know the answers to life, the universe, and everything. But, claiming that "God did it" and leaving it at that takes the fun out of observation, inquiry, and discovery.