1. Radio tower counts as an icon on the map yep!
2. You can use the same tower to decrypt all maps. Since that tile is a place you've visited before you would do steps (A-C-H-F-G) the event for the radio tower building would be decrypting the maps. So once you've done those yes, your day would end. (You can do all 3 in the same day)
3. Tiles with a treasure icon on them work like I demonstrated above. A-C-H-F-G. The 'event' is just finding the treasure no exciting text for those im afraid. The decrypted maps should tell you the terrain of the area you unlocked. Basically grab treasure and end the day.