こんにちは

1."弾く" make sense enough, but "弾きます" maybe more natural here because you use honorific in next sentence ("できます"). If you don't want to use honorific, you should change "できます" in next sentence to "できる". You should unify them (I know it's difficult. Never mind!).

私は機嫌が悪いギターにいるときに(、)私に(は)多くのことが(を)(ギターで)できます。

2.You can use "に" for connecting sentences. But using "、" may make your sentences more readable, and I guess "、" is easier for you to use, because "、" allows you to be able to separate sentences :)

3. You can use "が" here, but I guess "を" maybe easier to use, because you can use "を" for mark object in many cases ("case" here doesn't mean case (grammar terms) ).

4. You can use "で" when want to make words instrumental case ("гитарой")

また、私は風が好きです。

5. Good :D

The fixed sentences maybe still not natural so much, but they are grammatically perfect and easy for you to use... I hope!

こんにちは

こんにちは私は音楽を愛し、ギターを弾く（弾きます）

私は機嫌が悪いギターにいるときに私に（ギターで）多くのことができます。

また、私は風が好きです。