(section: 3 / sub-paragraph: ii / line: 2)
Correct. You can not estimate a digit on digital measuring devices.
(section: 3 / paragraph: c / sub-paragraph: iii / line: 1)
Yes, proper scientific notation will be an acceptable format for an answer in the Metric Mastery event.
(section: 4 / paragraph: b / sub-paragraph: i / line: 2)
No, you are correct. On a vernier caliper there would not be an estimated digit.