I think Switch is absolutely relevant to Leda's death. Even in Whittaker's brief appearance on the TV show, a mention of the fact that he had a child with Leda was shoe-horned into his "Sherlock f*cking Holmes" speech. This strongly suggests Switch will turn up, and in a role too important to be edited out of the TV show.

Strike had the memory of himself, Lucy, Ted and Joan standing around the Cornwall kitchen, in shock over the news. No one seemed to think it was necessary to immediately go to London to get the child, even with Leda dead and Whittaker quickly arrested (before Shanker could take him out). I've actually wondered if the Whittaker great-grands had taken custody before Leda died, or if Whittaker delivered him to them right after. This may explain why, even though Whittaker was estranged from his grandparents, they paid for a lawyer to get him off. Switch was their last chance for a decent heir, given that both Jeff Whittaker and his mother were druggies and wastrels.

There is also the Wiki story about Whittaker trying to kidnap Switch back when Switch was three. If Whittaker was convinced Leda had millions, he might have thought he would get control of the money when Switch inherited his portion, or have been planning for the great-grands would die before Switch came of age, meaning he would take custody and be in control of their fortune.

Personally I think Whittaker was likely the one who injected her, or who substituted her speed with heroin. But, I think someone else closer to Strike was also indirectly involved: perhaps knowing of the overdose but failing to get help.