Yup, and you received Experience appropriately. But Experience does not translate into Astral Rank without some means of conversion!
Yes, but the other options offer power as well. Now that you've bought Fell-Handed Stroke, the ability to use it with longer reach is quite relevant and in many situations offers more power than .25 points of Astral Rank. The ability to defer the cost of using it once per battle is also very good, especially right now when you are unable to use Pressure most of the time.
Well yeah, Seram had at least two major grinding sessions by that point and had also killed hundreds of level-appropriate encounters in the orcs! More than two, actually, since that was well after he'd started optimizing with training with Jeanne's assistance.
As to whether you could kill him, well, he still has kinetic Amplitude so your only real edge would be Accretion Rank and Charisma, since Evening Sky's resistance isn't immunity. Remember that he survived Witch Slayer attacks throwing axes at hundreds of miles per hour. I imagine he'd win almost every time unless you engaged him under very favorable circumstances. Plus, he's only got the one Curse to deal with.