Rusty isn't just a friendly mercenary who abandons his corporate contract for the good of Rubicon. He was the RLF's inside man.
Immediately after Operation Wallclimber, 621 takes a job to follow Rusty's trail of destruction, investigating RLF wrecks for communication logs that can tell them anything about Rusty. The logs suggest that there is more to Rusty than meets the eye, as some of the pilots say they know who Rusty really is and ask him why he would betray them like this. Rusty is already unusually forthcoming for a corporate soldier, warning us that the corps planned to sacrifice 621 during Operation Wallclimber. He also repeats a few times that Rubicon still needs him, which is a pretty sympathetic sentiment for a corporate drone.
We also know from Middle "Uncle" Flatwell's Arena profile that he has some old ties to the corporations. When Flatwell and Rusty show up for a 2-on-1 fight underground on NG+, the two talk as if they have known each other for some time.
- To be more specific, Flatwell's arena profile mentions that he has significant sway over a contact in Schneider HR specifically — the same company that "discovered" Rusty in the first place.
The end result is clear (or as clear as that bread crumb style of storytelling that From Software loves to use). Flatwell used his contacts to insert Rusty into Arquebus as the RLF's inside man.
But this all raises the question, why would the game's writers go to the trouble of including so many subtle and unsubtle details about Furlong when Furlong doesn't appear in the game? Simple world-building, or laying the groundwork for Furlong to have an expanded role in potential future content?
- Carla had herself implanted with Old Generation (Gen 1-4) augments, which use Coral. Several pilots who are confirmed to have received Old Generation augmentation surgery (C4-621, V.III O'Keeffe before he joined Arquebus, Sulla, and G5 Iguazu), which was only possible before the Fires of Ibis consumed the only known source of Coral, are still working as pilots fifty years later with no indication of experiencing any age-related physical or mental decline. So perhaps Coral-based augments possess anti-aging properties, and Carla is keeping the fact that she's augmented to herself.
- The developers' penchance for alchemy-related themes in their games supports this theory; alchemists viewed earthly coral as a kindred substance to the Philosopher's Stone, and coral was used to symbolize eternal life/youthfulness in medieval art. Carla's exposures to Coral may very well have caused her to age much more slowly.
- After taking the child Walter to safety in the Jupiter colonies to escape the Fires of Ibis per Professor Nagai's last wishes, Carla entered some kind of suspended animation (cryostasis?) for forty-seven years. She then awoke three years prior to the game's events, traveled back to Rubicon 3, and joined RaD intent on taking it over to serve Overseer's purposes. There's no mention of suspended animation tech of any kind in the game, but the pre-release story trailer showed 621 (who has Gen 4 augments and, as explained above, would have received them pre-Fires of Ibis) being revived from a state that looked like suspended animation to begin working for the adult Walter.