A plot hole is a plot hole, no matter how you look at it. You may find it "idiotic", but I think that it's enough to be a serious problem, especially for a mystery manga like Conan. Less than 1 year has past in that universe, but there are dozens of Valentines, winters, than summers, than winters again; and so on... Does it hurt Gosho at all if he just advance the plot along with the timeline ? (E.g. making Conan up to grade 2, than 3.)
Furthermore, it will be very incredible if the total time that takes Conan to defeat such a syndicate is only one year.