All right, intriguing news incoming.
After more thorough thinking and a brief correspondence with Harbinger, whose understanding of rules and their implications I do value, another problem with said ruling was unearthed. It was not only the fact that we could not settle on whether to have an advantage applied to the intercepted attack or not, but also the question of what happens when a boss starts using trash mobs as buffer? That is already a feature of Goblin Bosses, and it is a rather nasty feature to have at times. And then there was the matter of mounts, especially of the uncontrolled sort. What would be there to stop them always pulling off the intercept as long as they lived, protecting the rider even more than their additional movement already allowed?
In the end, all this simply added up to more headache than there was worth to be seen in it, so the proposed action had to be scrapped. However, it is not only bad news, for I remembered something. The usual Help action is, indeed, an action. However, there is a class whose Help is just better, that being the mastermind rogues. So it could well be that we could have a worse version of the protect after all. Choose one creature within five feet to intercept incoming attacks for, and apply disadvantage to the first one they suffer until your next turn. It is weaker action economy wise, as well as having less freedom of choice, but it would be an usable option.
Question is,
@Lucius Cypher, if this was the ruling made would you still seek to intercept incoming attacks, and if so, for whom?