Do I see it correct that the added line is only this one?
$ renpy.choice_for_skipping()
It is the only difference I could find in both bedroom.rpy files which is not in the lain mod file. Should adding it to the lain's verion of bedroom.py not make it a "merged" version?
If someone want to test it: (EDIT: added new file, no compiling error this time)
Some more details, please...
I got it to work by doing a clean install, playing for a few minutes and saving the game. Then I extracted the Naughty Tribute mod like normal. Then I played again for a few minutes and saved again and exited. Then I extracted Lain's mod. I then opened the "game" folder in Lain's mod and copied the contents into the "game" folder in the SummerTime Saga folder. That worked, I overwrote the bedroom file as suggested by others. It works great.
Before when I tried to do it, I extracted Lain's mod directly into the Game Folder in SummerTime Saga which then created a game folder within the SS game folder which caused it to freak out.
To summarize:
Clean install SS 18.6
Play for 5 minutes and save.
Extract Naughty Tribute directly into the SS 18.6 game folder.
Play for 5 minutes and save.
Create a new folder anywhere that isn't in the SS folder like My Documents in Windows.
Open the game folder in the extracted Lain's mod contents.
Copy the contents of that game folder.
Open your SS 18.6 folder => game
Paste contents in the SS 18.6 game folder
Overwrite the bedroom file
Make sure you don't have a second game folder in your SS 18.6 folder (this caused my game not to open)
That should make it work