ok I will say this to help as I also have not done the secret ending but when i was testing and trying for many hours i came up with a strategy as i accidently lost on like day 2-3 dont remember which and not with lily i am not sure how the events are after but it is possible to lose by lily on day 3 if i am correct and if you use the right items after that just go ham and try to get everyone as fast as possible also make sure to on the first day get rid of the red haired one aka use the meal
the blue haired one requires a necklace give the armor lotion (cant remember the name of the oil but it is type of oil) and if i am correct that was as far as i got on that i do have the feeling you also have to use a herb on the first day in someone's room hope this helps you
(also the strategy to lose to lily on on the third or second day is when you see lily in the dining room eat all the bread as each time you eat it for some reason it raises suspicion)