2 & 3) The Harmony supports activity startup and activity exit macros but these only run once. You can add delays in these activity startup/exit macros. The activity button mapping also supports sequence of commands mapped to a button. The current limits are 5 commands per sequence and 10 sequences per activity. However, one sequence can not call another sequence. The activity sequence feature does not support adding delays, but uses the inter-device delay settings.

5) All Harmony models have 2 modes. Activity mode and device mode. When you press the device button, you then select the device. Pressing the device button again will toggle back to the activity. You should be in activity mode most of the time. The device mode should be used only for debugging or accessing rare commands that you don't want to map to the activity to prevent activity button cluttering. Some Harmony models also have a Picture and Sound buttons. These are shortcuts to the Display and Sound devices used by that activity. They have their own button configuration independent of the device and activity button mapping.

This thread has lots of pictures comparing the various Harmony models: [Link: remotecentral.com]