A Seattle family’s luxury Cascade mountain retreat in a tiny lakefront community called Timber Cove is on the market for $5.8 million. Compass Brokers Bob Bennion and Mary Snyder have the listing. Bill Broadhead and Julie McCoy, owners of longtime Seattle-based political consulting firm Mercury Group, fell in love with the waterfront land right near Roslyn in 2010, thinking it would be an easy-access wilderness retreat they would build for themselves and their five sons. Located only an hour…