Bacon Bits

The character in OP's video isn't moving at the speed of light. He's moving so quickly that his [super] human opponents cannot observe him doing so. He's not moving at light speed. He's just moving faster than his opponents' eyes and brain can perceive. This is a biology question, not a physics question.

"There is no such speed in reality either." I strongly disagree with that. As I mention in my answer, the question being asked here is better phrased as "How high must a character's movement speed roughly be, to be quicker than the [human] eye?"

That analogy is a straw man. First, armor specifically doesn't work that way. Neither teleportation nor abjuration effects have such strict rules. Second, "protect" is not synonymous with "immune" (a defined term). "Protect" would use it's English meaning. Third, immunity and AC are not mutually exclusive. For example, werewolves are immune to physical attacks and yet they still have AC. Furthermore, if plate armor were described that way as the game's rules for what plate armor did, then actually you'd have reason for ruling that plate armor is ineffective against non-physical attacks.

Hallow? Yes. Forbiddance? No. Remember, there's no rules/crunch dichotomy in 5e. The second sentence of the first paragraph does not carry more weight than the first or third. Forbiddance says prevents all planar travel and therefore prevents all creatures from teleporting. The latter is a consequence of the former. An object teleporting would still be a form of planar travel of the type forbiddance stops.

Effects which hedge out teleportation effects such as forbiddance or hallow could prevent the bond from successfully calling the weapon -- more likely if the Knight were within the area of effect, but possibly also if the sword were in such an area depending on reading. Finally, nothing stops the dragon from simply destroying the weapon outright. The bond allows you to call it, but doesn't provide any special protection to the weapon itself.