Have you seen "The Bloo Superdude and the Magic Potato of Power" yet? It basically was the prequel to this episode, so having seen it may help one understand this one a bit better. The episode discussion for it is
here. 
In both "Bloo Superdude" episodes Bloo makes up a story. And knowing how egotistic he is, he is of course the "Superdude". The difference is, in the first episode ("Potato of Power"), he comes up with a crazy and exciting story as a way of telling Mac he destroyed his hand held computer game. In the second episode ("Celebration that he's not Invited to"), he's delirious with a fever, and this is what he's hallucinating as he's trying to get to Mac's party (even though he's suppose to be confined to his room since he's sick). Basically, they aren't really suppose to make sense, they're just suppose to be bizarre and funny. Because, of course, it's
Bloo who is telling the story.
True, neither are in the traditional style, but a lot of us did like the change of pace. I personally still liked the first "Superdude" episode better. You should try to get to see it if you haven't already.
As for Bloo giving Mac IOUs for a gift, and Mac redeeming them to fix his party, it does kind of refer to another episode- "I only have Surprise for You". Bloo has ruined virtually every birthday party Mac has had since he created him (and embarrassed him). For once, with Bloo sick and "confined" to his room, Mac's hoping for the perfect party for once. When Bloo ends up ruining it too, it makes sense that Mac will put down the iron fist and make Bloo fix it.
And as for Mac's age, I think most of us are decided he's now nine. I don't remember any reference to him being "eight" again after that episode aired.