Hi again Andrew!
You have asked several very detailed questions and I don’t have enough time to answer them all in one post, so instead I will answer the first 2 now, and then come back again and answer the rest when I’m able to.
Was it explained (either in the TV show) or book, why it's always dark in the year 2500?
I had wondered why myself, too. It was never explicitly explained why it always seemed to be night time there, but Nik did mention about how the sky in the year 2500 was “so full of gunk” that people were unable to see the stars any more. Perhaps this affected the sunlight too, although when Silverthorn and Draco met beside the canal, Silverthorn commented about the moonlight, so on that occasion the moon would have been visible (maybe it was a full moon that night?). And in the book version it was mentioned that the skyscrapers were so tall, they prevented light from reaching the street level.
Also, with the vast majority of the scenes in 2500 being filmed inside an indoor set, practically it would have been easier to create night time lighting effects rather than daytime.
-Was there any other point to the character of Lorien, other than they thought it would have needed a chaperone to get Jenny back to 1990? The way she was very quickly captured and moddied until the last episode showed there wasn't much interest in developing the character! A shame as she was interesting.
I agree, it was a pity her character wasn’t developed and spent most of her episodes in a moddied state!
As I said previously, I have suspected that Lorien was created to replace Tulista, but since I made that post it also occurred to me that:
-If the same storyline had been done with Tulista instead of Lorien, I think that Tulista would have been less likely to fall for Silverthorn’s pretence of having amnesia and of being harmless.
Tulista had the experience of being physically attacked by him in the first series, plus, after spending a month in 2500, Tulista would also have lots of observations of how people from that time generally behaved- lying, stealing and fighting- so would have lost at least some of her innocence and wouldn’t have been as naive and easily trusting as Lorien.
And even if Tulista HAD believed in Silverthorn’s amnesia act, I reckon the trauma of being attacked by him would at least have made her uncomfortable and jumpy around him. (That’s just basic human female instinct, and I’m speaking from experience as a survivor of domestic violence- as a woman, once you learn the hard way how dangerous some men can be, it rewires your nervous system)
-Being a technician, Lorien’s skills were needed in the final episode when she rewired the TimeGate settings to override Silverthorn’s setting its controls to his brainwave patterns so that only he could operate it. This was why the characters were able to use the Gate again to send Jenny and Petey back to 1990, but it wasn’t mentioned in the dialogue (I guess due to the episode time limits) but it was explained in the book.
Tulista was a historian, so I doubt she would have had the same extensive knowledge of how the Time Travel technology worked.
I’m thinking the main point of Lorien’s character actually WAS for her to be moddied, to show the audience how horrifyingly evil Globecorp was. They took a character who Alana cared about and completely removed all traces of her personality, and all those scenes showing Lorien walking around in a glassy eyed trance with no memory or recognition of Alana, and being forced to work with toxic chemicals, were to show us what happened to so many innocent people in 2500 and to remind us of the constant threat of what could happen to the other characters (Alana, Jenny, Petey, Nik, Silverthorn and his gang… and everyone else in that time) if they were arrested by Globecorp. (Silverthorn almost got moddied too, and Petey was semi-moddied.) And it was also a scary and sad scene when Alana attempted to bring Lorien out of it using her Transducer, but couldn’t. So I think Lorien’s storyline was intended to create fear and tension for the audience.
But I didn’t know the correct name for its shape until I searched online for a template to make 3D models of it. 
BIG NEWS!! 
