pwh the two vowels in “pity” are not the same (what sounds are, in different contexts, after all?) but they are (a) probably the same phoneme and (b) close enough for me, and probably close enough for the learners you describe. Certainly the “-y” is more like the second vowel in “pitiful” than the one in “please”.
That was what the OP asked, and that is the answer.
“Monday” is a case of vowel reduction. There’s a trend here too: nobody reduces the vowel any more!
Of course it doesn’t actually matter a hoot because the Taiwanese, like the “illegals” in Arizona, don’t distinguish between /i/ and /I/ in their accent of English. That’s nothing to do with KK, it’s just that there’s only the one vowel in Chinese (and Spanish).
The deficiencies of KK are to do with internal consistency, and the fact that it presents a phonemic inventory that is not representative of the English that any native speaker on the planet actually speaks 
I’d like to know more about the “implementation errors” you speak of. In what sense is KK being used as a second spelling, if that is the main or sole implementation error? (I’ve never taught kids, as you’ve probably gathered) Do you mean that the kids can use it as an alternative script when writing, as they seem to be allowed to do with zhuyin fuhao if they don’t know a character?
And Phonics? Isn’t that a form of weird spelling too? (Like you write “kar” for “car” or something, right?) But that is never actually used as a alternative spelling?
I’m trying to think how I learnt French as a kid: I started learning at school when I was eleven, kept it up, lived in France for a bit, and at that point I could sometimes pass for a native speaker. But I never learnt any phonetic symbols for French, or any pronunciation rules at all as far as I can recall, and it was only when I came to study linguistics, years and years later, that I found out that the vowels in “peur” and “soeur” are different, as are those in “son” and “sans”. And to this day I don’t really know how to pronounce what is normally a nasalized vowel before another vowel (“un agent”).
I suppose I just worked out the rules for myself, relying largely on English spelling/pronunciation as a hook. But Chinese kids who only know zhuyin fuhao and a few characters can’t use that approach. So what else can they use, apart from KK?
Key difference between me learning French and these kids learning English is that I don’t think I was ever expected to memorize or internalize anything. We just kind of talked and read stuff, wrote little essays… damn, I wish I could remember what we did. Certainly no lists of vocab to learn for a test.
It’s not KK. It’s not the attitude towards KK or the implementation of it, really. It’s the way they do education here; instead of learners learning and teachers teaching, we have learners memorizing and parroting, and teachers giving tests.
