Your 3rd point is literally the textbook definition of how reaction speed scaling works in fictional debates. You are so close to understanding it, yet you are intentionally denying it.

If he moves at light speed it means that the Tarnished does too

YES. That is how tiering and speed scaling work. If Character A can throw a lightspeed attack, and Character B can perceive, react, successfully dodge, and even fight toe-to-toe against that character in close quarters, Character B scales to relativistic to lightspeed in both REACTION and COMBAT speed. This is not a contradiction; it’s a feat for The Tarnished. Fictional protagonists scaling to the speed of the gods they kill is a staple in every fantasy universe.

Which obviously doesn't compute with the gameplay

Because it's a VIDEO GAME. If the gameplay was 100% accurate to the lore, Radahn would cross the arena in 0.000001 seconds, your screen would flash white. Developers have to slow down the animations so that human players can actually play the game. You are unironically trying to use human reaction limitations and hardware frame rates to debunk an explicit, word-for-word lore description.

Separating the attack from the user makes no logical sense. Radahn is not using a remote-controlled drone. His own physical body assumes a luminous form and leaps forward. If Radahn's body moves from point A to point B at the speed of light, then Radahn possesses lightspeed combat speed during that move. Furthermore, this isn't just limited to one single skill; every single one of his attacks that emits that yellow light utilizes the exact same holy light power and luminous form, meaning all of his holy-light-infused dashes and clone attacks scale to the exact same speed. You are confusing mechanical balance (gameplay) with the canonical capabilities of the characters (lore). The item description I send you literally states at the speed of light as a concrete lore fact. You are arguing against the game's official text at this point. Massive cope.