Yes, IF Raxxla is not a rogue planet* with superluminal speed capabilities, and there is a tiny bit of consistency left among FD "story tellers", then Raxxla is inside the Bubble.
Now, ofc anything is possible in a video game. Even the most ridiculous and disappointing inconsistencies.
A hidden storyline is an untold story, then no story at all. Usually, I base my assumptions on what we know and can logically deduce from it.
But, hey, after all.. what are the tin foils made for ? mmmh ?
No sirens in the Underworld, as far as I know. I assume the deepest void is witchspace.
*EDIT: sorry, not a "planet". MB told us that it is something different. Very convenient, in order to transpose it later anywhere they want, then.
EDIT-re: well... It still should be a certain celestial body, though.