In such cases as 1 (〜ing is....), you could translate like ''〜するのは…”(日本語を勉強するのは難しい). I think [〜 makes sb/sth ...] is a bit difficult to translate into natural Japanese sentences. One way to translate difficult sentences is to think of different ways of expression. For example, you could change the sentence2 into ''I feel happy when I play volleyball.”(バレーをすると幸せな気分になる。) Then you might be able to translate more easily. I hope my advice helps you :)

You could say 目を見る or 目を見つめる. The latter sounds more intense (means looking steadily).

Yes and no. Katakana is basically used for adopted/foreign words including people's name, countries' name and so on and onomatopoeic words. It depends on contexts whether it's frequently used or not.