[masaegu]

Dandan = gradually, increasingly. Who said katakana was used only to write foreign words with? I know you'd say "my textbook", but that is wrong. Besides, you are talking about song lyrics, which is in the same category as poetry. In creative writing, there exist almost no rules. This is not a newspaper article.

The 'toki' in 'kimi to deatta toki' means 'the time' or 'when'. Why in hiragana? For the same reason as above! The lyricist has that freedom in Japanese. And it's correct to use either 'to' or 'ni' in that phrase.

These questions show that you have been doing some serious Japanese studies, which is to be commended. Please remember, however, that language is a living thing and it loves all kinds of deviations from the normal.