Translated titbits from above.
Episode Aranea was being written by Koichiro Ito, who also wrote Episode Ardyn and previously worked on the P.T. demo.
Episode Luna was being written by Toru Osanai and Tomoyoshi Nagai of scenario company
Q'tron. Luna was also going to feature work by Yuichiro Takeda, noted for his work on all three Xenoblade titles (Also little secret revealed here, he was a plot writer for The Spirits Within but dropped out mid-production).
The plot for Episode Luna was all written when the axe fell. Takeda also had a scenario request for Episode Noctis, but it was cancelled before he fulfilled it.
PERSONAL BIT: Sounds very like the scenario would've been a lot more varied than in the novel. Four writers on one project, albeit working from the same scenario plan. Quite unlike one person doing it all with that same plan. Still, props to Nagashima for doing her best given the circumstances.