I can respect that translator's decision, but I can't agree on his reasoning. It's a portmanteau of two names:
Dumbledore: ダンブルドア
Gandalf: ガンダルフ
Mira's name: ダンブルフ ガンダドア
See, the ル from the bolded parts merged together and the remaining part of the names combine perfectly into ガンダドア.
In actual fact, there is no reason a Japanese will write the name as ダンバルフ because there is no バ anywhere on the original two names.
Unfortunately in English the name play doesn't really make sense this way, unless you write it as Dumblf or something.
Though I do think Dumbalf is the better choice here, because you are translating into English,
and just as it's not really right to use バ for the Japanese, it's not really right to use "u" in English either.