Understanding 「たった三十分のあいだに、高木の変わり様といったらなかった。」

Question

たった三十分のあいだに、高木の変わり様といったらなかった。

I'm not at all clear on the translation or nuance here. Source is a NLPT N1 grammar question.

Confusion #1: The use of に in 「たった三十分のあいだに」. If the translation is "in only thirty minutes", で seems more appropriate, or having no particle at all, based on Difference between 間 and 間に. All answers there indicate that に is NOT optional, and that it indicates a point in time rather than a long period. But the translation "at a point in only thirty minutes" seems like gibberish.

Confusion #2: I'm not clear on the nuance of 「高木の変わり様といったらなかった」. ~といったらなかった implies "very" or "incredibly", but with no other context, how do tall trees "incredibly change"? Their colors changing in the autumn? Their positions moving in a strong wind? I am not sure.

Answer 1

First of all, 高木【たかぎ】 is "Takagi", a common surname. (There is a word 高木【こうぼく】 too, but it's irrelevant now.) ～といったらない (or ～といったらありはしない, ～ったらありゃしない, etc) is an idiom meaning "I cannot emphasize ～ too much" or "～ is beyond words". I know it looks strange at first, but it's an idiom. So 高木の変わり様といったらなかった means "The way Takagi had changed was incredible". Maybe Takagi tried cosplaying, or heard very sad news?

たった30分の間に変わった indeed indicates that the drastic change happened somewhere within the 30-minutes period but the speaker doesn't know any more details. Note that 変わる is basically an instant state-change verb, so 30分の間変わった makes little sense. 30分の間で変わった is interchangeable with 30分の間に変わった.

Lastly, it's also perfectly fine to say:

たった三十分のあいだでの高木の変わり様といったらなかった。