Who said anything like a lead character like Harry Potter? LOL. How the heck did you get there? So someone has to be a lead character to nail a part?!!? You seem to be ignoring that Idris also didn't want to stomp on Will's portrayal either. What part of the character did Will not get right?
Your second point? Well I am never going to get the Patrick Star ending out of my head.....EVER. So remembering is not always a good thing. That might be the silliest thing I have seen on screen since the walking chicken leg house in Hellboy.
I'd add that the Starfish ending was made even worse by a joke earlier in the film about how starfish is another name for b-hole. So when the giant starfish shows up all I could think about was the team fighting a giant b-hole. That added to the absurdity and not in a good way.
I like movies that don't take themselves too seriously. That wasn't the issue. This one just failed to find the right balance.