I think a better comparison of the ability rather than shield is the dodge action (without the dex save bonus). Reworking the ability shifts dodge from an action , into a reaction, and requires it to target an adjacent ally.
Compared to say monk's ability to take dodge as a bonus action by expending their ki points. This seems rather powerful. As the monk ability uses limited resources, and uses a bonus action (which for monks is almost always used, where reactions are less so).
Also it doesn't really change one of your initial complaints. It might do nothing. If one creature makes a single attack against the target, it is effectively the same as before, and you have used your reaction.
So you end up where in some situations its extremely powerful. For example, following the leader in a narrow corridor. In which all attacks are now at disadvantage. And you end up where is effectively the exact same. Another issue: the Protector could use it, and the Protectee run away and still benefit until the Protector's next turn, which makes little sense thematically. So at the very least additional wording is likely needed.
Mathematically your change alters the effectiveness of the ability by 0%-500%+ (as 6 attacks in a round against a target isn't unheard of). It creates a wild variance in the effectiveness of the ability based on the situation. So any judgement on if it is "overpowered" depends entirely on how well the players exploit the new power. (for example two fighters could each take it, and watch each other so effectively the entire campaign attacks against them are at disadvantage)
Conclusion: The new power has potential to be abused/overpowered.
Suggestion:
I would suggest a different change. Allow it to be used AFTER seeing the attack, but still only affect a single attack. This means every single time you use the ability, you will at least be forcing a reroll of what you KNOW would have hit. So it would be more meaningful, however it would still only affect a single roll in a round, so it would not be a significant power jump.