(section: 2 / paragraph: b / line: 6)
No, underlining is not something that can be done on a PC standard 101 key keyboard by pressing a single key or a single key in combination with the shift key.
(section: 2b / paragraph: 1 / line: 10, 20)
Use of cartesian coordinate expressions as you described would be acceptable.
(section: Compet / paragraph: 1 / sub-paragraph: b / line: 1)
Yes, strikethroughs may be used to identify text that is not part of the completed instructions. Even though technically not acceptable since it cannot be made on a standard typewriter, students are not expected to perfectly write instructions and if mistakes are made, strikethroughs would identify that the text is NOT part of the written instructions ...therefore wouldn't be docked in scoring.