I'm just gonna get my tinfoil; and tell you that it's most likely actually just cop MILF mommy who texted that shit to MC, I think it's her effort to fuck with Miller, and she's using MC towards that goal, why else would she randomly give him an envelope filled with bunch of money and tell MC not to look inside, just for some text threatening to murder him showing up a few hours later. Only question I have is how Lainey got MC's phone number.
And yeah, Jenni is obviously related to whoever runs Freya, most likely her dad, she bosses everyone who works there around like they're her personal slaves.