
Is this a love triangle or a polyamorous relationship? Is there cheating ? Because I don’t really like these tropes …

It's neither of those things. The main character is still in love with his ex who recently dumped him to appease his dad who disapproved of the relationship. So MC studies hard follows him to his new high school for the elites, in hopes to get back together with him. His Ex however has forced himself to move on, because he has no intention of disappointing his dad.
The other guy on the cover is the dude who he admires, and is helping him move up in the ranks at the school to get his Ex's attention, but encourages him to rank up for himself, and not just to impress someone else. MC gives up on his ex, and this guy becomes the love interest for the MC.

I wonder how many years he spent in jail…

Wholly depends on whether or not the court buys this as being defense of others– if we're talking realistically, there's not a lot of hope they wouldn't charge him with murder (since there's little evidence), so that'd likely be 5 to 10 years, and up to 20 years considering the fact the body was buried and then Kaede went into hiding. He probably would've stayed on the low end of that sentence tho.
In the manga however they both look pretty much the same as before, so he might've gotten like 1 to 3 years for hiding the body only, and miraculously gotten out of the murder charge... It's either that or he got the minimum for the murder charge-- 5 years doesn't change THAT much of a person's face so it's believable I guess.
Art is peak but MC is clearly a product of Stockholm syndrome Lmao
him saying he loved him…no baby…that’s not love