Apparently, see dates below
2. When was the interlocker installed at Wayne Jct. and West Detroit?
Wayne Junction 1888
West Detroit 1890
Link about Michigan Central Interlockings
3. Why did they stop after crossing the GT at West Detroit?
The remainder of the trip to the Third Street Michigan Central Depot was apparently devoid of mechanical interlockings. So anywhere along there would be suspect. Bay City Junction did exist and didn't get a mechanical interlocking until 1900