Venezuelan opposition leader Juan Guaidó said he plans to return to his home country on Monday in a high-stakes strategy that directly challenges President Nicolás Maduro’s authority and could end with Mr. Guaidó in jail. “I announce my return to the country,” Mr. Guaidó said in a Sunday morning message posted on his Twitter account from Ecuador. He called for nationwide demonstrations against Mr. Maduro at 11 a.m. Monday that would be timed to coincide with his arrival in Venezuela. …