I am especially interested in fan theory on why shield doesn't work in subspace, and why subspace looks swirly.
1) Subspace contains an energy field that creates a destructive interference with the Shield.
2) Because its not the ships that are moving but the transit corridor.
1) Because the Lucifer needed to be destroyed somehow and they were not going to go down the "oh, it just takes lots of bombs"-route because that would not be approriate to the theme of "destruction of the old bringing forth a better new". If it were just a question of firepower the entire inclusion of the Ancients in the plot of FS1 would have been redundant.
2) Because it looks unfamiliar and is disorienting; yet also its motion translates to the idea that the Lucifer is moving when it cannot for gameplay reasons (otherwise the Ursas would take forever to catch up).