Do they come back to life? Or does it end with one of them dead?
Hmmm... They do "come back to life" in a sense that the dead person does reincarnate after hundreds of years. They'll meet again in this person's second life. But if you're talking about whether they come back to life during their first life, the answer is no.
Does it end with them meeting again or we get to see they develop their relationship in this second life?
Sadly, it ends with them meeting again. It's highly implied that they'll fall in love with each other all over again though (im just assuming based on google translating the japanese raws). So it's more of an open ending iirc. It was only like 1-3 pages where we get to see them meet again. Obviously, the reincarnated person doesn't remember the other so it's quite bittersweet.
Urgh I hate bittersweet endings, I think I’ll drop this now. Thank you for telling me tho
Tbf, I despise cheating but like given the context, I'd say the top is an asshole for doing that but he's also a victim.
He was obviously forced to transmigrate to another world with the sole purpose of killing the demon king. He didn't want to do that, so the only other choice was to marry said demon king to get back home. Obviously, he didn't want to marry the guy because he thinks he's straight but between the choices of killing or marrying him, marrying the guy was the "tamer" choice. It was either murder or marriage in order for him to get back to his original world.
Obviously, he doesn't exactly fall in love w/ the demon king at first sight because he's "straight". He also blatantly assumes that their marriage is just a mutual business transaction and that it's just a marriage "on the surface". Remember that marriage is a mutual decision but this seems more of a forced marriage situation on his part. He's scum for cheating while he's legally married to a husband who tries, but y'all aren't getting the full context. They aren't a "couple", hell they weren't even friends, they're just "married on the papers". It just sucks because Rei (demon king) really did try his best to make him happy and wished for the marriage to eventually work.
Also spoilers, one of them dies. And I mean that the death isn't because of old age.