I'm pretty sure whatever he has with the AD was never pure love by any definition.
That said, while what happens next is probably obvious, I do find it interesting how openly he's been trying to avoid this, and still is. But he finds himself unable to simply because he's afraid of making her...
Hyung isn't a name, it's a word for "big brother" used by boys/men. It's used here as a term of closeness/respect for people older than him. The hyung he met at the wedding is not the hyung he talks to at work; they are different older men he is close to (or is sucking up to). Incidentally, this...
Unrelated, but I always find it funny every time someone casually mentions that Suhyeon is the most handsome person in the group ("don't upstage the groom!") and also he's like a head taller than every other man in the story.
I took it as them having dated before they started working there, maybe in college or something. All their old friends assumed they would still be together. They seem way too physically comfortable around each other to just be formerly close coworkers.
So his "never did this with a coworker"...
Mangadex does not have magical insight into the nature of the story, nor is there a backlog of untranslated chapters someone might be looking at to determine that. The tags are added by the uploaders based on their personal opinions, and their opinions aren't any more important than yours or...
Unlike in manga, polyamorous marriages don't always work out! It's probably not a good idea to be so casual about it. I know someone who got poly-divorced and it was messy.
I would describe it as the women in this story generally get more interiority than the man does. It's not leaning 100% in either direction, but in a scene where they have a conversation we're more likely to hear the AD's inner thoughts while we're left to judge the man by his words and actions...
For reference, the author's previous work had the guy "dating" five or six girls at once, but this never caused any problems because somehow no one ever noticed. Then each of them got a separate alternate reality ending where he married one and pretended the rest never existed. Honestly it was...
It's essentially a form of "limited edition". If something is perceived as rare, then people will line up for it even if they don't actually want it that much, just because "I might never get another chance." Not to mention the people who go there hoping for the rare item who end up buying...
That's not why he concluded that. He concluded that because he saw how deeply relieved she was to learn that Lemon wasn't dating him, and since he also believes Kogahara hates him (because he friend told him that), so by process of elimination she must be happy to learn that Lemon is single.
To be honest, the dream scenario here (entertainment-wise) is that he completely gives up on Kogahara and spends his time in Lemon's body trying to push her together with Lemon because he thinks they both secretly have a crush on each other.