I understand that an antibody screen can be used to detect a certain type of pneumonia, among other conditions. Can it also be used to identify various infections in the way that blood cultures can?

Yes but. Usually an elevated IgM response to a particular organism indicates an acute infection. However doing IgM ab to a whole panel of bacteria is expensive and many of the tests are not available. The culture method is the default test but can be false-negative in many cases. In the future, I believe PCR analysis of bacterial DNA or a bacterial DNA microarray assays will likely be the way to go.
Yes. This is particularly true for VIRAL infections! Hope this is helpful! Dr Z.