
Why is the older brother so obsessed?

For the same reason why all narcissists and abusers are obsessed with their victims: easy mark to control means easy way to continue to pretend to be a normal well-adjusted human. They lack confidence in themselves and view themselves in a poor light, by having someone need them, they feel better about themselves and by abusing them, they “get rid” of the negative feelings. They put a lot of effort into perfecting the art of abusing someone just enough to keep them satisfied and love bombing them just enough to keep the victim around. It’s a hard process for them to learn since each individual is different. They end up in a codependent state to their victims and think of themselves as better since they would “pick up the trash” a second time. It’s classic abuser psychology.

"forced romance" as if Tess hadn't been here from the beginning.

Does Tess have any character ter depth outside of being a love interest for the MC?
Does she have any screen time when it's not MC related, or trying to make her look like the perfect girl for the mc? Where she's just treated like the average supporting character? Do they have natural chemistry or do all the scenes with them together feel like the author is like bending the world to try and make it seem romantic?
An example of forced romance is jinwoo and his girl. An example of NATURAL romance is jinwoo and the ginger healer girl, the reason they're more natural is because the author doesn't introduce her with the sole purpose of her being his damsel in distress, but for her to be a stepping stone and just another bg character, so the author isn't trying to bend the whole solo leveling universe to make every scene romantic, as they start out as friends.
The BEST example of natural romance is bromance in action, fight me idc but that's because the male characters aren't intended to be together, their bond is real and not forced as they are genuinely just the MC and a supporting character who happen to get into scenarios that build their bond. With the female characters, it feels like she was only ever introduced into the story to be a damsel. That sums it up

I'm just so disgusted by the ML's "parents". They just wanted a nanny, a nanny who would follow their son 24/7, someone they could manipulate. I feel so bad for the ml knowing he never had a moment of peace.
(I'm not defending his actions done in adult years, I just feel so bad for the baby/teenager him)
Everything was so fine until the fl became stubborn on pairing ml and ogfl. Like girl he isn't a toy. He has a thing called brain, he CAN THINK. What the hell even goes on authors' mind when they make fl forcefully meddle like that? So annoying
Specifically in this one it is more about feeling guilty about the ogFL faith and less about "because this is what supposed to be in the original story), she just can't think of a different solution to help the FL. Usually it is "only the FL can make him happy blah blah he will definitely fall for her blah blah"
I agree that it is annoying though, and in my opinion the only reason they introduced the crown prince was just for that
Imo it only gets annoying when they drag it out even when they both clearly already have feelings for one another. Most ppl would feel like a third party in this situation I think. It'd be weirder if FL was okay with it and decides to steal the ML right away. It'd be a "villainess" origin story. She'd become Trashta 2.0 ╥﹏╥