Yeap. You were one second away from what you wanted. From your save file 21, wait until midnight and eavesdrop on Kate and Thomas. Choose "This has to end!", go to Kate's room to talk about the blackmail and that's it, you will have the options to give her wine. Next day you can talk to Thomas and keep the love route or allow Thomas to punish Kate (NTR route).
oh man i never wanted to chose the option " this has to stop" because i though it would definitively stop the ntr route with thomas, especially after i saw this in the guide :
" At 20A you can tell Thomas to stop blackmailing Kate if you help her in the evening. (If you want to go into NTR with Kate read the NTR part below before doing this) "
see how you get stuck if you don't chose an option that shouldn't be a prerequisite for the actual quest ( what's the relation between "this has to stop/blackmail" event and " drink some wine in order to get the dance and dress stuff " event ? none in my opinion, also completelly counter intuitive when you play ntr.
thanks very much for your patience and help. i have a complete save of the game with every scenes but i wanted to play the story from beginning to enjoy it and i must say i was ready to stop playing this game it just made me rage after i tried to find where the problem is for 4 hours.
thanks to Pax too for helping me.