Of course Becka was going to go off the deep end! These girls have been part of "salvagings" for so long, that violence is the only language they know! They've seen men punished for their deviant behavior, so Becka did the only thing she knew and punished her father!
I don't know if this is technically a spoiler, since it's just a theory, but I've been thinking about who Garth's handler could be.
I think it's either aunt Lydia, or someone connected to aunt Lydia.
Aunt Lydia knows everything about the girls, she's been paying very close attention, so she knows Becca isn't super happy about her future as a married woman, and she also said that having friends is dangerous, so what better way to drive a wedge between her and Hanna (aka Agnes) than having a boy come between them.
Book spoiler ahead:
Also, if Lydia is part of mayday, like in the book, then she probably knows everything about Garth and may even be his handler, so she paired him up with Becca because she can see Becca slowly getting disillusioned with Gilead, and Garth could turn her into a mayday asset.
Does anyone think he's one of the victims of the Jezebel's poisoning back in season 4? Remember the scene with the "Sufragette" song where all the Jezebels were giving the commanders shots of alcohol laced with (I think) Nightshade? When Garth said that, I immediately thought that's how he got poisoned. Then again that was way before the Boston fight.
So I have a theory about what could happen.
Daisy will reveal to the aunts that she has her period SPECIFICALLY so that she will be sent to the dentist, but she will make a huge fuss about him touching her, maybe even scream so guards will run in and see him doing something. His family will be purged, which means Becca can't get married now because of her low standing, and if they stick to the book, aunt Lydia will protect her by making her an aunt. Daisy will instead be married off to Garth because she has no rank, and he's the lowest on the totem pole. Boom, mayday couple will help bring down Gilead.
Lol in all seriousness, I don't think that'll happen but that's my guess.