Formal. Kiwamaru and kiwamarinai are exactly the same meaning and can be used interchangeably. Kiwamatte is also the same meaning, but is used as a conjunction and is often used with the word感 (かん, feeling).
Mainly used in written Japanese.
Although 感極まる and 失礼極まりない are often used in conversational diction.
I don't understand ex #3317 and the corresponding translation. Ex #3315 seems to be from [kiwamarinai] entry, but perhaps one entry is enough for all the examples.