
SPOILERS
For the ending, was that the black haired dude, saying "I want to tell you my proposal even if it's the second time" or something? And when he said, "I don't mind how many times you say it", what exactly was he referring to? Can someone explain the ending to me. Thank you!

In believe he was referring to the night before when they were both drinking and the teacher said he forgot half of the night but you can tell from his blush that he remembered the part where they confessed to each other. So by saying that he's going to tell him a second time, I think it means he is going to tell him again while sober what he had told him the night before. To show it wasn't just drunken talk.

Hold the phone. What exactly was the third misfortune?

A monetary misfortune. The karaoke machine was 1000$ or something like that, and got broken. They mentioned that the misfortune didn't have to be aimed at living things. They even distinctly mention the karaoke machine in this page:
http://www.mangago.zone/read-manga/saiyaku_wa_boku_o_suki_sugiru/mf/v03/c019/21/

When Morinaga tells his Senpai, that Morinaga's family is not as understanding as the Senpai's dad, can someone remind me or point me to the chapter where Morinaga meets the Senpai's dad? I don't remember this. Or I read the page wrong: http://www.mangago.zone/read-manga/koisuru_boukun/mh/manga/koisuru_boukun/v10/c002.2/pg-18/

Can someone remind me of how mutants are made again compared to regular vampires?
Spoilers-ish Question
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Also, I forgot. Why is it that the main character, Mano, didn't turn into a vampire after being bitten by Carvel?

So, turning into a mutant is just a random chance. It's not like they do something special. Which is why Carvel was surprised Geron turned into one. As for the second question, them being bitten/exchanging blood seems to be a way to form a contract. And turning into a vampire most likely requires the vampires will to do it. Since it doesn't really show him doing anything special to change Geron.

I don't like how this author has characterized Hideaki as some neurotic, mood-swinging, dangerous gay character. I'm pretty tired of this trope honestly. It's not good representation.

It would be interesting if the character ruined the relationships in a more comedic and stronger way. Like if he owned up to it instead of hiding and becoming unstable towards the one he loves. I think there are better ways for relationship-ruining to be done without making Hideaki the dangerous, unpredictable, and scary gay character.

Hey! Is this shounen-ai at all? Apparently the two main guys kiss in this? Is that true? (≧∀≦)

Yeah, it's Kaworu and Shinji who kiss (I'm pretty sure it's Shinji's first kiss). And about the shounen ai, I guess it's up to interpretation? Kaworu doesn't show up until the last chapter of volume 7, so it's a while before they meet. I felt that Kaworu liked Shinji romantically because of volume 10, and it's not until volume 11 that Shinji admits he was attracted to Kaworu, despite Shinji thinking that boys shouldn't like boys and telling himself that he didn't want anymore friends.
The anime does have more moments between Kaworu and Shinji that could be interpreted as shounen ai, but I thought the episode was quite rushed. The manga also has moments that can be seen as shounen ai too though. Whichever you choose though, like Kaworu said, but there's not a happy ending for them, and the series itself is very sad.
So much better than the original manga. This spin-off is hilarious and adorable.