While there is quite a bit of intense and complex wordplay in The Phantom Tollbooth, I firmly believe that this sophistication makes the work more interesting and inspires greater curiosity in the English language. This language may stymie young readers at first, but I think it inspires them to pick up on new words and phrases they would otherwise not be exposed to so easily. The use of wordplay instills a sense of whimsy through repetition and quirkiness, which is interesting enough to teach these children those phrases and words. Juxtaposition between the normal and abnormal is something the book sets up along the way, and so children should easily be able to appreciate being asked to fill in the blanks of meaning on their own: "Expect everything, I always say, and the unexpected never happens."
For example, reading the phrase “Whether or not you find your own way, you're bound to find some way. If you happen to find my way, please return it, as it was lost years ago. I imagine by now it's quite rusty” uses repetition and rhythm to both confuse and delight; the punchline of the joke also uses wry humor to convey the point of the wordplay. These elements combine to create a sense of fantasy and complexity that does not hold itself above its child readers, but asks them to engage along with it, and find out what the words mean along the way. Can this philosophy of asking children to engage with and find meaning in books and their language go too far? Is there a line in The Phantom Tollbooth in which this occurs, where the wordplay goes over the top, to the level where readers 'check out' of the book?
You make an interesting argument for the value of words over numbers (simply due to their sheer amount of use), and the Brazilian tribe that has no numbers is fascinating. However, unless one lives with that tribe, numbers still maintain a useful part of our society. While they are not used as often as words in terms of volume, they arguably carry a greater level of importance per usage - we need numbers to count our money, to know how old we are, to figure out car speeds, count the things we need, etc. Numbers, per number, are more important on average than words; words must combine and be formed into phrases and contexts in order to have proper meaning, while a number says it all.
I think you are very much onto something with the character of the Humbug; while everyone else acts according to their name (the facade they put forth), Humbug frequently helps people out. This could indicate, for one, that unlike Rhyme and Reason (who are exactly as they say they are), Humbug is actually a somewhat helpful creature. There may be a bedgrudging lack of acceptance of his true nature; he may wish to name himself the Humbug, but this desire does not match who he really is. A 'humbug' would not wish to impress as much as he does; he puts on a personality that does not quite fit him. He does always wish to impress, but it is almost a desperate attempt to maintain this personality that leads him to help on Milo's quest.
Juster, Norman. The Phantom Tollbooth.