She did have a plan, she just didn't account for Cercei throwing caution to the wind and murdering dozens of nobles.
Her death served a much bigger purpose than that. Without it Tommen wouldn't have killed himself, likely destroying the last shred of Cercei's sanity and paving the way for her to become the Mad Queen.
I'll agree Benjen was much more than fan-service but they really didn't have anything special for him to do. He can't go through the wall and as far as we know there's nothing of particular note for him to tell them.
Jon is the last living son of Ned Stark, a hardened warrior, a leader of men, and honorable to the core. It's unusual but not improbable considering all that Jon's done.
Jon fought beside his troops in that doomed cavalry charge, he was one of the first into the fray when Smalljon Umber attacked them from behind, He led the attack on Winterfell's gates and personally caltured Ramsay Bolton. He may not have won the battle but considering his actions during it, it's no wonder he gets most of the credit.
Jon has proven himself to be a man every bit as brave, honorable, and just as his father and brother. It's because of the man he chose to become that he was crowned king, bastard or not.