Uschi is Martha's daughter (Opa's cousin), who had escaped to England. Uschi likely had no knowledge of her parents' deaths, or even that they had gone missing.
I don't know who Helga or Frau Herz are, certainly don't know where they are. I'm guessing a long shot that maybe Herz is referencing Hirsch- which are the extended relatives that Ella has who live in New York. There is no chance now of her leaving Germany to go to the US now that the US and Germany are at war. Maybe she's asking if they've been in touch so that he can rely on them for help if he needs it? No idea.
Hanna is a relative that I think Opa might have mentioned seeing, or Ella is trying to urge him to contact out of familial connection.
Again, a short postcard from Ella leaves us with more questions than answers. We know she is alive. We don't know what her living conditions are, or how it is that she is still living at her apartment with Jews being shipped out by the thousands. All we know is that for one more day, Opa can take a bit of a breath. As always, she is "Yours, Mama."