Ok lol, calm down buddy. First of all, "あ" is "Ah" not "Oh", so both "あんハピっ!" and "アンはぴっ!" read the same.
With the mix of hiragana and katakana, there could be a few reasons. 1. Fujisawa is being creative in making an interesting title, 2. he's making an emphasized, clear divide between "Un" and "Happy" for whatever reason (might have something to do with the spelling, 3. Simply, it's very probable that it changes the way one would read it, either putting more emphasis on "Un" or "Happy". Sound effects in manga often cross between katakana and hiragana to give different effects.
I'm actually guessing the reason is a mix between all 3 points.
Anyway, yeah, the small tsu at the end of the word. Hehe, I'm guessing you don't look at much manga in Japanese? Next time you're flicking through your Bilingual GTO books watch out for it. In short, a small tsu at the end of a word is pretty much a Japanese version of an exclamation mark. Kind of. In essence what a small "tsu" does is tell you to cut off what your saying abruptly. Basically it's telling you that the "pi" is meant to sound really short. Wondering why there's an exclamation mark after the "Japanese exclamation" mark? Emphasis man, emphasis
