1: Either grow a pair and kick them out, kick out the players who invited them, or just deal with a large party (7 isn't too bad, but you have to dial up the challenges about 1/2 a CR over the large party penalty in my experience). Alternatively you could find another GM so you can split the group.
2: Rewrite the story to avoid trope, lampshade the tropes, or subvert the tropes
3: Final bosses should satisfying when you beat them. Use every dirty trick/trope you can think of.
4: You can make it work, but if your party is genre savvy you have your work cut out for you. I strongly suggest breaking up the party and keeping your half lower key, perhaps even abandoning the "save the continent" story line.
First: Breathe.
Second: I've rebooted my own homebrew 3 times in 5 years; v1 and v2 both only made it to 5th level. I COMPLETELY empathize.
To the extra players I say have a conversation with them. Tell them you're flattered but overwhelmed and see if maybe they want to split the campaign; the ringbearer and his trusty squire head for the volcano while the rest of the party makes for Rohan to gather the disparate tribes of man... or something like that.
But seriously level with them on how you feel. They might surprise you with resolutions you hadn't considered and you owe it to them to run a game ALL of you can be proud and happy with.
This plot seems like its very large, very unwieldy; lots of moving parts in 6 kingdoms. The party's only 2nd level. Maybe for now concentrate on smaller "chunks", say spanning a couple levels at a time:
. 2nd to 3rd level they find a spy for one of the 6 kingdoms that wants to put a kybosh on unity.
. 4th to 6th level the party follows the clues from the spy back to the kingdom, finds that the kingdom is split into 2 factions and the one that's in charge is in league w/the ultimate villain. Now they need to defeat them in a way that doesn't make them enemies of the other faction.
. 7th to 9th puts them on the trail of a new kingdom to swear into the alliance - if they were successful in the last arc and their reputation preceeds them it might be easier initially, but to secure this new kingdom they must restore some kind of honor through a quest or series of quests.
...and so on.
Notable enemies: the spy could be a trusted ally or even a family member. Also I like using bosses that foreshadow the ultimate villain. My first campaign featured a giant horde for the potential final villain so I used a pair of thawns and a few ogres as outcasts from the giant's society. My second one was based around the feud between 2 ancient evils - a dragon and a ghoul queen. I used kobolds, split into 2 tribes - one mutated by magic with black-green scales that lived among the swamps and some classic ones that were necromantic zealots.
What kind of game is it, what kind of theme? And your players; tell us about them. What do they like/not like about the game? Does anyone else seem stressed by the developments?
I wouldn't pull the plug, not yet. Talk to them and then decide.