Mmm, I believe you are correct.

"One of the impressive" is phrase that is perfectly fine when one is talking about multiple restorations and they are all impressive. But then, "this realm has ever seen" is added to the end of the sentence which changes the requirements.

"You all had─and'll continue to have─a hand in one of the impressive rebuilding efforts of the realm." would be ok.
"You all had─and'll continue to have─a hand in one of the most impressive rebuilding efforts this realm has ever seen." would be ok.

"You all had─and'll continue to have─a hand in one of the impressive rebuilding efforts this realm has ever seen." isn't correct.