It's the same as the difference between volitional する and non-volitional なる.
e.g
言葉にしない not to say it intentionally
言葉にならない(= 言葉にできない) can't say it/can't describe it in words
こんなん is a colloquial form of こんなの, not 困難, there. こんなん refers to こんな占いの結果, thus, it means something like these kind of fortune telling results comes out.
Incidentally, that's a popular fortune teller's stock phrase, probably 20 or 30 years ago. She always said this phrase at the end of her fortune telling.
The meaning is almost the same as 短い時間で, but 短い時間の中で sounds that the speaker is thinking about a specific time period which is already settled.
… often represents a pause.