We have a winner! 1. Three is correct. In May 1980 they began to add improvised lyrics to it live & it evolved into Forever. By the end of the month Three disappeared completely from the set & Forever took its place.
2. The master tapes for Seventeen Seconds were badly damaged after spending years in Chris Parry's utility room, on top of his tumble drier (allegedly). Hence A Forest needing to be completely re-recorded for Messed Up. For the deluxe, a pristine original pressing of the album was used to create a remaster (which would also account for the better dynamic range).