
Can someone please explain, what exactly happened to the brother? Why is MC trying to get revenge on these powerful people? What did they do and *why* did they do it?

Artia the guild where the brother is from was betrayed by his friends and lover from the same guild. The MC who received the delivered pocket watch from his brother had a kind of flashback of what happened to his brother including all the important points to do in the tower. The MC wants to take revenge to those people (friends and lover who betrayed) who stabbed his brother's heart. It's not yet pointed out why they betrayed him, but in the recent chapters one from those people who betrayed the brother, he was apparently a spy from another guild
Hope this helps (づ ̄ ³ ̄)づ

Is this fully enjoyable without ever reading Dark Lady? I kind of have no intention of reading that one, but this Origin sounds super interesting.

If I'm going to be honest I don't really know if my answer will help you in any kind of way but I myself haven't read Dark Lady and I'm still enjoying the story.
Yes.. ofcourse there are some things that I don't like about this but you'll always find things that you don't like in everything. So I don't pay too much attention to them.

Can I just say that I love how the author ACTUALLY uses good business sense, accurate timelines, and real economics for the story? Don't get me wrong, I really like a LOT about the story, but I really really appreciate the details and sense that the author has (so many authors use... Very unrealistic methods). GREAT worldbuilding for Hines territory!
Examples of details I appreciate:
***There are SPOILERS after this point****
1. It actually took 6 months for him to fake his recovery (and yes, if he'd been in a coma for 6 years without healing capabilities, it would take SO LONG to recover), and it 4-5 months to grow the moongrass (no insta-growth).
2. He didn't flood the market, he was smart (so many authors ignore the fact that flooding the market will drop the price!!!)
3. He thought about the best way to revitalize his territory long-term, in a way that actually makes sense! And would work!
I would read 10 volumes of this, it's amazing.
Also, I really appreciated Seiya's character, and I found I empathized with his worries a lot - as someone who loves others but has a low sex drive, I really understood his struggles (especially in vol 2). In this, you could tell he really loved Kiriyuu, but wasn't sexually there sometimes - this has definitely happened to me. I also appreciated how the seme handled it; that is, that loving someone doesn't need to equal sex all the time, and that both people should want it.