So, I haven't find these fics unfortunately, but while looking for the second one (about Buffy being the reincarnation of a slayer) I found this challenge which fits perfectly. Maybe it will ring a bell...https://dark-solace.org/elysian/modules/challenges/challenges.php?chalid=405
I found more clues on a post
by World Words on FanFiction.net (July 2017):
«Hello, i m searching a fic that i read last year i think. i will tell all i remember about it, if you dont understand what i m saying, its my fault, not yours, i totally understand english but stink when its about speaking it... good luck and thank.
i dont remember if this fanfiction is completed or not (even if would say she is finished, but not sure )
The story begin just after Spike was turn, he met a strange girl who fight demon...The Slayer. At this time is name was Betty Winters (i'm not 100% certain of it, but i remember the Winter Summer ) Spike and her fall in love. But one day (just after they sleep together i think) she is kidnapped, raped and murdered by Angelus (in front of Spike).
Many years later, Spike found Drusilla, kill her and learn that Angel is helping a slayer and in love with her. He doesn't accept that. Want revenge and go to Sunnydale to kill... Angelus's Slayer.
So that night when Spike observ Buffy fighting at the Bronze, things change a little... she have the same face, the same fighting skills,... than his slayer (betty). she is her. reincarnation so He save her.
After that i dont remember well, except that Buffy will less and less like Angel and fall for Spike. specially after Halloween where she disguise herself in... Betty. and with the Ethan curse, she become for a night yhe ancient Slayer, gaining all her memories.»