GRAND RAPIDS, Mich.—Prosecutors preparing for the first prison sentence in an alleged plot to kidnap Michigan Gov. Gretchen Whitmer are loudly signaling to five other defendants that a key insider has shared extraordinary details about the operation. Ty Garbin cooperated within weeks of being arrested, willingly putting a “target on his back to begin his own redemption,” the government said in a court filing. Prosecutors want U.S. District Judge Robert Jonker to take that into consideration Wednesday when he sentences Garbin for conspiracy. The government is recommending a nine-year prison term, a long stretch but one that would be even longer if he had not assisted investigators after being charged. The FBI last October said it broke up a scheme to kidnap the Democratic governor by anti-government extremists who were upset over Whitmer’s coronavirus restrictions. Six men were charged in federal court, while others were charged in state court with …