still不作连词，可以作形容词、副词。所列的例句的still, yet基本都是副词。
第3句建议用but或yet
still: （来自柯林斯词典）
ADV 但是;尽管如此;(虽然…)还是
You use still to emphasize that something remains the case or is true in spite of what you have just said.
I'm average for my height. But I still feel I'm fatter than I should be...
从身高来看，我算是中等身材。但我还是觉得自己胖了点。
ADV (表示不必担心)然而，不过
You use still to indicate that a problem or difficulty is not really worth worrying about.
Their luck had simply run out. Still, never fear...
他们的好运已经到头了。不过，别担心。
yet: （来自柯林斯词典）
CONJ-COORD （用于引出颇令人惊讶的事实）可是却，却又
You can use yet to introduce a fact which is rather surprising after the previous fact you have just mentioned.
I don't eat much, yet I am a size 16. 我吃得并不多，但是却穿16码的衣服。