If the gap at the bottom of a metal door is more than 3/4″, can the deficiency be overcome by adding an automatic door bottom or a metal astragal-like strip to the bottom edge of the door?

Refer to ANSI 250.8 paragraph 2.1.8 entitled “Design Clearance” and the subparagraphs underneath that. Those paragraphs specifically and clearly delineate and define the design clearances for between the door and the frame, between the meeting edges of pairs of doors, and the clearance measure from the bottom of the door to the bottom of the frame.