But I agree Sierra and Sahara are way too close to Savana, you'd be tongue tying all the time.
I would try to get him to like Margaret or Margo. That is strange that he would veto naming a daughter after your mother IMO. Even if he thinks it's old fashioned or something. I have a nephew named after my dad, his name is Roscoe, which is WAY old fashioned and strange IMO. We call him Rocky. I just see Roscoe as an old man's name and so did my brother but my dad was going through some health problems before Rocky was born and my brother thought he was going to die so he named his last son after our father. He even broke his "A" tradition. His other 3 kids all have names that start with A like him.