Katakana/Kanji is imperfect, which is probably why ZUN always renders names in Japanese and English, so that there's no ambiguity in how they should be spelt in English, or pronounced/spelt in Japanese. Unfortunately our alphabet language is phonetically ambiguous compared to hiragana/katakana. There can be multiple 'correct' pronounciations.
Both Alice, Alyse, Elyse, etc, can be rendered with the Japanese "アリス?, there really isn't a way to differentiate (even though they would be pronounced differently in English). And well, if ZUN didn't write Flandre's english name, she could have been Frandoll, or Flandouru, etc.
So, are you the type of person who accepts multiple possible non-Japanese pronounciations, or believe in one absolute correct pronounciation with the others being strictly wrong?