The existence of the Giant Spider may come off as confusing and random to many viewers. But think back to earlier: we've seen a fly the size of a football, a weta the size of a small man, and a worm the size of another short person. What do these characters all have in common? They're allBig Creepy-Crawlies! So of course the spider would have to be that big. That way, it could eat the aforementioned characters in a manner not unlike a real spider. After all, spidersare generally bigger than the bugs and worms they eat.
That guppy that Bletch ate and later vomited up on the golf course? HE WAS ALIVE IN HIM THE ENTIRE TIME.
Robert prevents Trevor from killing Heidi, allowing her to kill Trevor and Bletch. While nobody will mourn their demise, Robert had no way of knowing that Heidi wouldn't continue her rampage and kill even more people, potentially including himself and his love interest - especially given that, in stopping Trevor from killing Heidi, he indirectly put himself in harm's way.
The Muppets Mayhem confirms that Meet the Feebles is canon within the Muppets universe.
Additionally, unless we assume Broad Strokes was applied to the plot of the film within the Muppets universe, Peter Jackson's comment about two of the Feebles being in witness protection, while the rest were in prison, implies that most, if not all, of the Feebles who survived the massacre may have been victim to a Happy Ending Override.