a. Good question. 「～～に限って（は）」 means that "~~" is an exceptional/particular occasion, person, place, etc. 今回に限っては = this particular time. It is emphasizing that what happened this time has never happened before.
限る may not be too difficult for Japanese-learners to use when it means "to limit" or "to restrict".
However, very few of them are able to use the verb in sentences like:
「屋台料理はフォーに限る！」
「夏は冷たいビールに限る！」
限る means "to be best" in these phrases.
b. It is used for total negation or denial. "it was absolutely not a 誤解"
c. Exactly.
