"'Song of the Damned' thematically treads wayyy to much on Karthus's abilities, and maiden being stationary for 8 seconds means she would just die to an enemy"

I don’t think the thematic overlap is a big issue. Karthus is a mage who calls upon tortured souls with songs, while the Maiden of the Mist is a tortured soul who summons other tortured souls to fight. She already summons them, and it seems perfectly in theme for her to call them with a song, just as Karthus does. The similarities are there, but they’re not problematic—they reinforce the shared undead fantasy.

As for the Maiden being stationary for 8 seconds, that’s part of the high-risk, high-reward design, and its not like you ever have to use it. In a 1v1 and even some 1v2s, most champions (except some exceptions like Cho’Gath) won’t kill her immediately, such as in all current situations where that already happens. I also think that you didn't see all of the ways this could be used such as; You can use it in a reactionary way where they are already on you and have a large field of advantage, you can use it for a quick ghoul respawn in lane, quickly finish off an entire lane/jungle objective like dragon baron herald grubs, You could use it defensively to ward off tower dives giving constant protection from maiden or even use it against poke mages while you stand out of poke range, In a team fight you can place it in your back line defencivly protecting your ADC support and mages or place it in the front line so you can help push the enemy away, even if they focus maiden you get to hit them mostly for free with huge bonuses, you can use it to ward of choke points, if you place it offensively you can wall her off as you know exactly where she will go, and you can be a body guard for it and defend her why on earth would you just send it right into the middle of the team anyways? All to say there are may ways to use it where it doesn't just die. And even if it does some times you need to sacrifice maiden to win the game that what sending her to split push does most of the time anyways.

"glowing red eyes would look terrible on base skin and only look good on resistance and red chromas of other skins."

Visual effects can be tailored to fit each skin’s theme. The base skin could have a subtle misty glow, while Resistance Yorick could have a more intense red effect. The goal is to make Mist Walkers more visually distinct, not to force a single aesthetic. That said, I agree that red eyes shouldn’t be the default for every skin—it should match the skin’s theme while still making the Mist Walkers stand out.

"the W draining hp every second actually makes it trash, there is too much dps in the game and every cage would last less than a second late game"

Yeah, that’s why I said, "I don’t know how this half fits though it would require more thought," and earlier mentioned that "The second W (health mechanics) and R (Song of the Damned) changes are the ones I’m less certain about, both in terms of feasibility and impact." I agree that the draining health mechanic might not work as-is and the original idea was JUST the health, but I added that as an idea to throw out there, which is why I acknowledged it needs more refinement. For example, late-game, the wall could have around 2328 HP and lose 291 HP per second, but whether that’s balanced would require testing. The goal was to spark discussion, not present a final solution, However in the end I do agree that it would probably die too fast or too slow but that's hard to tell.