First thing's first. You can use (spoiler) text (/spoiler) to make spoiler tags. Just use [ and ] instead of ( and ).
The mega tier secrets are:
1. Obtain one "final" ending. (Whichever variant of Ending 1 your playthrough gives you.)
2. Finish Soltis and get the prize that's waiting for you at the top. (Do this to obtain Grandmaster Kaoru in Soul Game as well.)
3. Get the Champion's Medal.
4. Get the True Champion's Medal.
5. Get the Big Key on the beach and return to the Grand Library. (This one is seriously purposely obscure. There's one major hint for it but I by no means expect 100% of all players to finish this.)
For your other things, I'll leave them itemized as you had them:
1. It belongs to Karnelia, so the Heart Container is the actual reward there.
3. You have to get the final main story's dungeon item and two other items to get inside Soltis. (The bird hiding behind Iliss in Plergoth has a hint for this.) If you don't want to bother with the hint, skatz25 is right about previous posts.
5. The monument is just kind of there for a cameo / inside joke. If you're looking to fill the Artifacts area in your subscreen, there's another feather you can get from an earlier area in the game. (You can return there later if you missed it on your first go.)
8. You need the Soltis item at the top in order to be able to get the dangerous weapon in Sanskrit as well. The key on the beach...I'll only offer that there's a Secret Shop with a map-like hint for where you can stumble upon the secret. Other than that, you're about on your own. (Refer to my earlier comment on it.)
The Mirror Tunic:
- It's sadly just an ominous tease for now. The method I had in mind to script this thing was absolutely terrible and I abandoned it, but it'll still have some significance somewhere in the series hopefully. The lore behind it is all still legitimate.
- If I can ever get this thing working, I may come back to Eiyuu and give it its own side quest. Otherwise it'll show up later in the series.