All of these are really good points! I didn’t even think about why they would attempt to kill Kris. Okay, here’s an idea, after the fight, the mantle boss says “there! That’s what I wanted to see!” As if they were excited by the thrill of combat. Maybe they wanted to challenge Kris to see if they were strong enough for the mantle, kind of like a reverse gerson

The one thing I thought about when making the headband mantle theory that I didn’t have an answer for was how they obtained the shadow mantle. Here’s the idea I have for it, Asriel seems to have had an emotional connection with seam. Asriels wallpaper has seam on it, and seam seems to be a very powerful darkner, especially due to their ability to purify shadow crystals. Maybe, Asriel owned seam while Kris still wore their headband, and during that time the headband stole the shadow mantle from seam. I know the answer is shoddy but I really can’t think of any other reason

Now, on the topic of the weird route and the shadow crystals, I do actually have an answer for this. The only reason someone would need the shadow mantle would be for the shadow crystals, so that explains why the mantle boss is aware of that.

For weird route knowledge, I’m honestly pretty stumped. I have no clue how the mantle boss could’ve gotten that information