Have a very firm set of headcannons for the ending of Sanctuary For None which include Helen leaving Nikola to set the self-destruct up for her so all she needs to do is activate it, and that Nikola's job for the explosion itself is to get Henry free of the blast radius and he doesn't want to, but he does as she asks. He doesn't realise until she kisses him and they do that goodbye my love stare across the lab that she doesn't know if she will make it out.
After the blast, Nikola digs her out and shoves her into the underground Sanctuary swearing at her the entire time. She let's him fuss. Henry stays above ground to 'hunt' for the body and then, devestated ofc, goes to London as a decoy and slips into underground that way. Kate finds out when she arrives to Hollow Earth with the Fifth Ward abnormals to find Nikola Tesla slouched in a chair muttering about being Helen's errand boy. Will had to wait till he'd quit SCIU, and he isn't impressed he didn't know first until Kate points out that Magnus told her son and boyfriend first and he needs to get over himself. They all find Biggie getting his strength back in Magnus's new, fancy infirmary.
Other headcannons percolating but genuinely just want to think that Helen, at the ninth hour, told Nikola and Henry enough to let her blow the Sanctuary up but I bet they didn't actually think she'd do it, hence the devestated shock on their faces. They thought it would be a just in case bluff and are afraid she didn't make it out of the fight with Caleb. And that her immediate family - Henry and Nikola - are who she has to the new Sanctuary first. Imagine how pissed off Will would be to arrive into the Sanctuary to find Henry going 'Dude! About time!' And then Nikola waltzing in, calling him the Prodigal Protoge or something.
Leaving that as my happy place. No more angst thanks. Teslen forever after, Helen and Henry's mother and son duo.