Dianne is a device for the wake up call for Lacey, for sure.
You see it in pretty bleak terms, I seen people recovering from abuse by changing the "scenery", that is obviously not the only ingredient, but for Lacey, MC is one of those ingredients.
Lacey willingness to get over her sexual hangup is clear, in my view, at some point she would have tried to overcome it even with MC.
If MC were there she would not have sought validation in that way, she would have probably sought isolation with him and only him.
MC would have needed to try and open her up softly, like he opened her up to eating chocolate.
That said this is all speculation of the highest degree, so it comes from our ideas and personal experiences, those might not reconcile.
I am willing to listen to you if you have a different opinion or you see things in a different way.
Since it is you I promise I won't pounce

.
The motivations behind Pimp behaviour are these:
MC_nvl "She thinks that people aren't genuine enough"
MC_nvl "And that people aren't their true selves until everything gets turned upside down"
MC_nvl "She does it because she thinks people can't be happy if they aren't who they were meant to be"
So she has her idea of what is genuine, and if I were Lacey I would be VERY diappointed, considering that she thinks the genuine version of Lacey is a gutter whore with no self respect (the pimp refer it as glorious) and she just pushes and manipulates people in that direction.
No matter who they are, no matter what they think, no matter the emotional and mental damage that she can cause.
Writers fail sometimes, he probably wanted to create this misunderstood, good natured and somewhat naive person that tries to get the best from people and as a result he wrote an abusive monster.
He needs to go back and retcon some stuff / give her a VERY powerful redemption arc, where she recognizes her mistakes, ATONES, separates from the group understanding the damage she has done and then maybe be accepted again.
We don't know where the bag is, we don't know where it came from and we will probably won't know unless it is tied to familiar voice or used as a plot point later.
The pimp doesn't have money to get a new bikini but she does to get at least 6 grams of K, which in my country if you get it cheap would be around 200$ give or take, from internet research I see that in the Us it would have been more than double.
Anyway, you both discount what we see of the relationship between the pimp and her bitch.
The friendship is strong, for some reason they act like comrade in arms and they do not manage to stay angry at each other.
This is especially true for Lacey, she forgives and forgets in an extremely wasy way.
She, despite everything, tries to see the good in people and in their actions.
I know this will trigger you as you both despise her profoundly (well
Maviarab has more of a love/hate relationship but he is pretty no nonsense with her), but in this case I don't see anything different from what Lacey usually does.
When the pimp exposed her past I would have expected a pretty gruesome confrontation, we don't see anything, because the pimp got forgiven instantly and the "sharing" plan was not even on the table then.