The whole mother going missing, Abigail finding her and ends up raped is the same.
What change is after the cumming.
Original event was that she kept on raping her, and Abigail recalls her mother's story of people who's been turned and her mother's command to kill her before she fully turned.
In the happy end, after cumming inside, the mother comments on how she feels refreshed after letting the stuff out and says she's going home. (Which is kinda amusing and ironic in a way, the reason she raped her daughter was because she was trying to hold back the lust...)
Months passed and when the Alchemist visit, the conversation flow along this line:
Mom-Still can't believe she ends up pregnant
Abi-Well, since you do it every night
Mom-But if I don't let it out I'll turned, you don't want that do you?
Abi-Well....no...
Mom-(whisper) I'm getting horny again
Abi-(whisper) Wait, now? There's a guest so can't you wait?
Mom-(whisper) "it"'s coming out~
Abi-(whipser) oh fine
Abi-Sorry about that Mom is not feeling well so we'll end this here
Alch-Oh don't worry, have fun
*proceeds to go from fellatio into sex, off screen*
Alch-...Happily Ever After....I guess?
(The specific word was mededashimededashi which is usually says at the ends of a tale)
Also note that after achieving this ending you'll have the sound of the two having sex at the starting screen.
