To get Jessica and Dave together you need to do 4 things:
1. Get Eva and Dave to break-up, this can be done in a number of ways, generally the idea is to have Eva get corrupted while also pursuing Dave sexually, at some point you should be able to tell Dave Eva is cheating on him, alternatively you could date Eva yourself, that will make her break-up with Dave regardless (but there might be a couple of issues, more on it in a bit).
2. Make Jessica turn good, be her friend and pick the blue dialogue options when talking with her, encouraging her to quit drugs and make up with her parents.
3. Stay wearing the good outfit into the endgame.
4. Either be single in the endgame, or never had lesbian sex with Jessica. This might be caused by bugs but if you are in a relationship with anyone and had sex with Jessica at some point in the game, the conversation about Dave won't trigger (so if you got Eva and Dave to break-up by dating Eva, you might need to break-up with Eva by taking her to the poolparty and getting her addicted to bbc). If you are single, reject Jessica's confession and then you should have the option of telling her to pursue Dave.
I don't think there is a scene with Jessica and alpha Eric, maybe there is one with beta Eric but I don't play that route much so I don't know.
