I'm only up to the beginning of season 5 but this is what I know so far

1) no but he's dated only women
2) maybe the pilot if you want but you can really drop in anytime, "two guys wackily solve crimes" is all you need
3) once so far
4) I like the american duets episode, the one where they go to a haunted house, and the one where shawn is in a telenovela