I believe it's possible to have the A/T in ARM mode (other than during idle descent), however I can't seem to achieve this. When I press the A/T disconnect button it drops offline entirely.

 

What am I missing?

Just press the MCP button corresponding to the current active A/T mode (SPEED or N1). It will deselect that mode and the system will revert to ARM mode.

Excellent, many thanks!