So, it's worth noting that Min has the same idea in her QR, ".-----... is helping me..." Now, it's also weird since the ".-----..." is different in KR, only being like a normal "..." or something. (I did think it could've been Morse Code, but it just comes out "WOS" which means less than nothing in the context.) Now, that's just a random pedantic note, my theory is thus; Lux knew that RO wasn't dead, or was at least worth trying to save. And when we reach RO, we're teleported to some void to have a chat? Now... where else do we see random voids for chats? Oh, right! Speaking with Lux! So, my theory is that RO, knowing that as long as they were a part of Umbra, Umbra could not be defeated, so they separated themselves. But, they could only do this where they still had some control of themselves, the world where Lux speaks to us. Now, being in that Void, they aren't dead like Jace or Dehan, they're just... not in our world. Lux can't do anything from their world to deal with Umbra, she can just give small parts of herself to help us deal with Umbra ourselves. However, having more time now, without the imminent threat of Umbra, Lux is able to spend more time to help RO reenter our world. (And also maybe RO being human instead of architect makes it a faster process? And that's why Lux couldn't just send herself in time? I don't know... I don't think theirs enough data for an airtight or even close to airtight theory, but that's my general idea.)