In order to get Promiscuity Level 5, you need to engage in a lot of events / do a lot of stuff which are level 4 Promiscuity, in terms of events you have the park's restroom's at night which take half an hour, and during combat you can do a little trick against one single enemy, which is you can give them two handjobs, one footjob and a blowjob and it will perform the check on each of these starting from the top and get the first successful one. Aka, if handjobs show a Very Easy level of difficulty, you can use every sexual moves against that enemy and it will increase your arousal, their arousal, your sexual stats AND your promiscuity level at the same time ( even if the move wasn't succesful / another move already got used )
You can come to the brothel any time, and you'll actually be able to come across Leighton on Friday and Saturdays. If you haven't blackmailed them, everything will be fine and you can actually ignore them ( or comply with their demands, which are actually slightly less risky than normal NPCs ). But if you want to improve your stats and don't mind gaining 65% less money but wish to avoid getting raped and your character losing control, the Strip Club does that very effectively ( just Scream if a patron is getting a bit too comfortable ). Leighton doesn't show up at the Strip Club.
To blackmail Leighton, you need to sit in Maths class and follow the class until you're tasked with delivering an envelope to the headmaster. You can check this envelope ( requires a bit of Skullduggery and will make Leighton spank you at the end of the day, or even steal your clothes and spank you if you did other delinquent stuff during the day ) and check the Headmaster's Office while Leighton isn't there ( break in the school between 16:00 and 21:00 I think, but I know it works if it's 17:00. To break in in school, you either need to discover the hidden path to the park ( requires level C of History, go to the rear of the playground and you should see an option to take a hidden passage ) or a secret passage from the toilets to the drain ( require level B in History ) ). Once you've checked the computer, you can come back to Leighton's office at any moment they're inside ( during school hours ) and blackmail them. Additionally, you can go to the police station and report Leighton's misdeeds ( unlocks one one-time revenge-ish scene against Leighton and gives you some control back )
In order to unlock the Robin Debt quest, you need to have Robin's Love to 100 and their Lust to 10, then sleep in your room, go to Bailey's office and check the new prompt before going back to your room and go back to Robin's room.
As for any means of revenge on Bailey, it seems that Bailey's well too-connected in town to be delt with significantly. At best you can be a minor inconvenience to them, but they'll be sure to make your life a living hell. Also, no deaths will happen in this game.