At what point does the Autopilot/Autoland stop using Glideslope for guidance?
It depends on the aircraft. On the 747-400, for example, the glideslope is abandoned at flare height which is between 40 and 60 feet, depending on descent rate. The autopilot aims for a touchdown with a descent rate of 1.5 feet per second (radio altitude), 450 feet past the glideslope transmitter. On the 747-400, localiser is used during rollout, perhaps down to 40kts (retuning of receivers is allowed by airplane logic below this speed)
Like the other aircraft types, there is assistance provided by the inertial reference system. For example, when the G/S is in use, ground signal loss can be tolerated for certain time periods (e.g. 10 seconds). A sensible "inertial path" is maintained. Rapid deflections of ground signals due to, say, other aircraft interfering with the signals, are recognised as such (and the aircraft doesn't go crazy trying to follow the signals).