Jolina doesn't need grand role, she might be down on luck friend/lover who needs money very badly.
What could the motivation be though?
a) Dizzy's victory condition hinges on the MC leaving Monica and family or not
b) the victory condition is more about romance, maybe to be in a relationship with the MC or to get pregnant (which also means MC will almost certainly leave Monica and family and get money) - if Bella is Dizzy this scenario seems a bit more likely, i.e. she only gets money if MC is with her and leaves Monica et al. and maybe Donald wanted to create extra incentive for MC to leave Monica and family by having Bella throw herself at him (could be his twisted way of making sure his son does get the money)
What role does that leave for Jolina?
Scenario a) MC has to leave Monica (even if that means Dizzy has to share money with him, but as mentioned maybe asshole Donald wanted to make double sure the MC is not with Monica and family and gets the money)
Jolina romancing the MC makes sense, but why would she be so abrasive and downright hostile at first when he first asks her out? That could have seriously discouraged him. She would be a lot more welcoming of the idea, she would fawn over him. And why would she involve Jenna with anything? That makes no sense.
Scenario b) MC has to stay with Monica (Dizzy may potentially get all the money)
Jolina would be incredibly stupid to romance the MC, he may leave everyone for her, Bella would also be colossaly stupid to pursue the MC, so it's very likely Dizzy's victory condition hinges on the MC leaving Monica and family or at least to be in some kind of romance/relationship/maybe be pregnant with his child, given how she acts towards the MC
And the Beach House is already confirmed to be 100% the MC's property, no matter what, the lawyer explicitly confirms that the 100k, the car and the Beach House will be the MC's, no matter how things with Monica unfold. So I find it unlikely that Bella's victory condition hinges on the Beach House or the MC letting her stay there. Which would also be a very lame condition, the other two conditions we know about come at a great personal cost. So maybe Donald wanting Dizzy to be the MC's girlfriend / mother of his kids could be that personal cost.
But if it's pregnancy or to be in a romance with Dizzy... Jolina would not help at all, unless Jolina herself is Dizzy, or maybe Donald had a threesome with her and Bella and both are Dizzy. If Bella is Dizzy then what could she gain by her ally (Jolina) romancing the MC? Unless her condition is to simply make sure the MC is not with Monica and family, then having an ally could make sense. But why would Jolina then first shoot the MC down when he asks her out on a date and why would she involve Jenna with sexual activities, maybe encouraging a throuple with MC, Jolina and Jenna?
If Jolina is involved I'd really like to know what her angle is, why she is involved and why she behaved the way she did. I cannot see how her involvement makes any sense here, given her behavior.
Unless Jolina is more out for revenge against Bella (if she's Dizzy) and the two had a falling out (and she knows about the will to begin with, she knows a few things about computers, maybe she hacked Bella). But how would Jo exact her revenge? Helping with the email to Monica makes sense if Bella's condition is to get the MC away from Monica and maybe to be in relationship with him (both works). But what about the romance? Why would Jolina romance the MC? Unless it's genuine of course and she really likes him and as long as Bella is not with MC Bella loses.