Can stream entry be reached by focusing on the links in dependent co-arising
It depends what you mean by "focussing on". If by that you mean thinking about mentally, then no, that is not possible, since the mind will be focussed on an abstract object and not be able to see impermanence, suffering, and non-self.

If by that you mean observing empirically then yes, since the links are ultimate reality. By focussing on them, you will see impermanence, suffering, and non-self, which is the path of purification, according to the Buddha.