So!~
After a million attempts, trial and tribulation - I managed to get the secret ending.
For those that wish to have the experience of observing the outcome of managing to get Adam to fall for the MC, here's a link for the save file.
Enjoy!
PS: Thanks for the hints Mr. Creator, they were more than helpful. (though I was a bit dense at first and kept trying to cram more time with Adam, not focusing enough on Carlos) The ending is cool though.