Why did the county’s law enforcement begin to suspect that a team of two people may be behind the arsons?

The law enforcement found footprints of two different shoe sizes and shoe prints at the scene of one crime. Also, the properties were hard to get to by car— an arsonist would have to be traveling by foot through the woods, but an abandoned car would be easy to spot in the time that the arsonist was walking and inside the property. Someone would have tipped off the police if they sighted an empty car waiting near an abandoned property. Thus, they surmised that someone else must be driving the getaway car in order for both of them not to have been caught early on.

Did the prosecution know about the surprise character witness before he was brought out? How did they ascertain his identity?

No...