Using indefinite ("a") instead of definite ("the") article with "key to success"
Solution 1:
It is absolutely valid, yes, for exactly the reasons you say. "The key" refers to a specific thing, "a key" means one of many such things.
Although we often read "the key to success is (whatever)", obviously there are many important things that lead to success, so I look on such statements as dubious.
That said, in your example sentence, I feel like it would be stronger without the "a", and simply "Timely breaks with friends are key to success."