This is some nice guesswork!
I had similar thoughts here and there but:
Lacey's big secret that she can't hide much longer...
- I do see the possibility Lacey dies... whether by suicide or a terminal illness... Some of Lacey's actions resemble someone who knows they're going to die, and is doing all they can to "fix" the people, in this case one person, they really, truly care about (see Leap of Faith)... The person knowing death is coming does this so that the people can be happy, move on after they're gone. Lacey doesn't want to just "abandon" the MC again while he's not well, and she doesn't want to "leave" without the MC's love and forgiveness... However, there's also some clues, hints that cause me to seriously doubt this guess... But right now, it's one of my two best guesses...
She often says she wants to be with him forever, build a family etc, so this can't be it imo.
If she had HIV for example which can be kept completely asyntomatic and not contagious thanks to medication it would be super obvious anyway, plus I understand llying but that would be like the lie of all lies.
What I can confirm is that lacey wants MC to be happy with or without her but she also needs to be with him 100%, I hope these two diverging thoughts make sense.
- I saw some comments that Lacey may have a kid... Possible, but not seeing the indications or hints anywhere that this is the case. I could have certainly missed them though. The most likely kid possibility is that Lacey is pregnant... She can't hide that for long. And of course, the drama is/will be, is it the MC's, Damian's, or Morty's, if the reader went that far...
Other than that, I can only go into wild speculation territory...

She has a twin. She's not really Lacey, a doppelganger or twin... You know, wild card crazy twists that come out of no where...
Missing birth control due to being fucked up and be pregnant with Damian's child is like....WOOOW.
I also don't think she was pregnant before, even if it would fit my "Lacey's divine comedy" headcanon and would be fun for me, I don't think it would match the dialogues AND the moment where Lacey hints about the other side, why bring that up after the Mc blowup? Maybe because she thinks MC discovered the lie? hm i don't know but I feel like it is more tied to her being without him bringing out the worst in her.
Other than Lacey's big secret, who is Monster?
- My best guess, Christine is my most likely (there was a subtle hint that I'm trying to locate again...lol), Jeanette is second (working alone or with Christine), given their IT backgrounds. If they aren't alone on it, they are certainly working closely with one of the "crew", or with all of them...
- It could also be Mia, she referred to herself as a monster quite a bit, and she could be using Christine to relay messages to the MC indirectly, so whoever (likely Barty, could be Jared, too) is manipulating Mia doesn't catch on or "win".
Any way, that's what's swirling around in my head.
I'm remaining flexible on this story going forward, not developing expectations or anything, but very much looking forward to see how this plays out...
The Monster writes in a very detached way, at some point I tought he could be Jared on a redemption path, but with Bella's messages? I don't know, plus he knows Anna.
I would love you to locate the hint from Christine, because her actions were weird when they talked about the video.
Even then I can't reconcile what and how TM writes with her.
Can't be Mia as he received messages while she was in the pool with the others, so she either snuck some messages there or I don't know.
Veronica is the last one to join everyone, so she could be it too, but the messages arrive later.
Naw she doens't even remember Isaac very well, TM is insulted when being called Isaac.
At the same time TM doesn't "know" or want to use Mia, Anna and Christine names.
It couldn't be someone from the crew, as we are told by Anna that none of them knew about Mia's plan.
This also points against my favourite theory, sadly very flawed that TM is MC himself, some sort of split superego.
So anyway, a lot of speculation on my side, please find that Christine hint!
Carefull now, i think she would see that as a reward instead of a punishment.
Mostly i agree. But she knew that it was something he didnt want and yet she made the lesson happen. She knew the whole situation would hurt him, sure she did not think it would be that bad but... She took the drugs to excape her selfblame and in a way avoid responsability. And then read the text and ignored the self restraint in favor for her pleasure. Choosing someone she knew and works with, even the way she talks during the scene, there is no phrase on how sex means nothing to me or similar, and is all about "look at me, MC, feed me." All that plays more in a conscious or instictual attempt to feed than in instant gratification, which i accept also happened. She, like in college, didnt care enough in that moment to even consider MC, his wishes or desires, she abandoned him for a moment if you want to think that.
By the way i joined that brotherhood.
Well I am glad you joined, from now on I think I will refer to her only as "the pimp"
I know we are quite close in our thinking, but it is a matter of nuance.
You say that Lacey knew that will hurt MC, in reality she think that would help him, it is so deranged as a concept that it is hard to accept, I know.
But she did that because in her head that is what was needed to assure him that no matter who or what she fucked or fucks, she would ALWAYS be his.
She ignored him to force that on him, like she is forcing the open relationship concept.
Also she took drugs because she did not want to do it, she needed them to go through with the thing.
Obviously everything in this majestically fucked up plan failed and ended up huring MC is ways unimaginable.
And yes in that moment, no matter how much she says "you were there with me", "YOU made sex pleasurable" (actual quotes btw), she didn't give a fuck about him, the only thing on her mind was her sordid plan and her pleasure.
The drugs do not make you innocent, else when you get into a car accident you would just say that you were high to avoid any consequences.
On this, you are absolutely right, and the gall of saying "it was pleasurable because I was drunk and YOU made sex pleasurable for me" is absolutely fucked up.
The real way to hurt Lacey is a ban on red bra nights for a while , that should teach her
Edit:
Ok I am a bit of a sadist but that is a whole next level.
Fucking hell man, seek help

(jk)
Edit:
Made herself a mama in college and kicked the child out of her life, thats my theory. With MC abandonment issues, if he discovered she abandoned something as important as a child he would be triggered, plus it would be another thing he is not the first to do with her, more triggers. With the timeline we know of her in college it could be possible, but just a theory at the moment.
The problem with that theory is that the toilet phase lasts only Junior year and we have detailed pictures of the whole timeline, 9 months is 9 months.
UNLESS she got pregnant with Isaac and her sickness we hear on the phone was she being pregnant, MC has to wait one year before seeing Lacey again so there would be time either to abort or to bring to completion.
There isn't much pointing to this theory so I don't think that happened but if it did it would be a final nail in the coffin.