So I've beaten this game twice now and I really, really don't get the endgame.

Cereza IS Bayonetta in her childhood...so how'd she get in this time? And why was it so important that her pendant block Jeanne's dagger (which randomly turned her good?)? Honestly, this hardly ruined the game for me since I wasn't really into the plot from the start and still LOVE the game, but I'm just curious if anyone can tell me what they were going for?

Yeah, I had the same issues. The only thing I can think (stick with me here) is that Father Balder (Baldur?) created young Cereza to get Bayonetta to come to the tower, and in doing so, some of Bayonetta's life force/power/whatever actually transferred to Cereza, which is why he absorbs her for the fight.   
After all, he sends her those 'my dear child, I am watching over you' messages all the time, so he's got to know what they're up to. He also says something about creating her to get Bayonetta to the tower, although that's quite far into the cutscene, and I can't be bothered to watch it again.
It's also possible that Cereza was simply part of the father in the first place, so he's putting her back to regain his full power (although that's a very creepy thought).
 
And, to be honest, I totally missed the bit where her pendant blocked a dagger. So you can probably ignore me.

I think the young cereza was actually beyonetta that Balder timekidnaped. He did use her to lure Beyonetta to him knowing that she wouldn't be able to remember the little girl was actually herself. Then once Beyonetta figured it out the truth, she was able to see the pure version of herself uncorrupted by the world. Thus balder was able to open her left eye against her will...etc.

what's also weird is that bayonetta says "i've seen that girl before.." and then she gets flashbacks of when she was in the jail cell 500 years ago next to young cereza and she sang lullubies to her and gave her a pendent 500 years ago. so i guess that means father balder sent young cereza to bayonetta 500 years ago? could that mean little cereza has been running around vigrid for 520 years??
did anybody else go "that lady in the cell is bayonetta! how can bayonetta be locked up in the same place as her young self 500 years ago so it's her mom! but no that lady is of impure blood! so she IS bayonetta! so that means she gave her young self the pendent 500 years ago and that's how she has it now?? or was it from her mom?"

LoL its amazing that I haven't played this game I come into a thread with End Spoilers and I still don't understand anything about the story.

" LoL its amazing that I haven't played this game I come into a thread with End Spoilers and I still don't understand anything about the story. "

Balder brought his daughter, that being Cereza/Bayonetta into the current time in order to lure Bayonetta to Isla Del Sol so he and she could awaken Jubelius. Jeanne was working for Balder and the Lumen Sages and was there to help Bayonetta regain her strength/memories; she was always on Bayonetta's side. Balder says that Jeanne was rebelling against the Sages in the cutscene before you fight him.

She needed the jewel that Bayonetta had.