Most likely she will find some Baratheon cousin with best claim and make sure he is loyal to her, before dealing with garrison inside. In reality, as soon Stannis is confirmed dead, Cersei would have to get the castle soon otherwise she would seem weak. So Storm's End is similar situation - probably title is empty in the show for simplicity reasons, we don't know for sure. Normally Cersei would move in just so nobody else can - why would you simply let Dany get a foothold so close to Westeros? He would also leave garrison on Dragonstone (and he did in the books) but I assume it is too much for the show so they have to keep it simple -> therefore Dany simply walks in into empty Dragonstone. In reality and in the books, he would leave small garrison to keep Storm's End like he did himself during Robert's rebellion. Now, all Renly's troops means for sure all Stormalnds lords and Storm's End troops - Stannis is the only surviving brother and therefore ruler of Storm's End and Stormlands. In show it is not stated but implied since he met Renly near Storm's End and all Renly's troops switched sides. Both in books as well in show, Stannis took control of Storm's End.

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