Is it possible for destiny to exist within the realm of happenstance and vice versa?

Answer 1

Yes. Check out Carl Jung's writings on synchronicity.

Answer 2

I don't believe destiny necessitates the absence of free will.

If you can imagine circumstances where decisions/actions, once made, negates the possibility of alternate decisions/actions/outcomes, then you have answered your own question.

For if, by virtue of free will, situations evolve which direct outcome, then destiny becomes the sum of the possibilities of the result of that application of freewill.

For example, if you murder someone, you have eliminated all other courses of interaction with that person which would involve them being alive. Your destiny becomes that of a murderer from the moment you decided to commit murder, and not before, when you still had opportunity to leave that person living.

For the person murdered, their destiny was to be murdered once they decided to, (say go buy something), and happened upon you. Their excise of free will also dictated their destiny.

For the more there are beings of free will within range of each other, the more the prelevance of 'happenstance'.