What should the nickname for Estela be?
I think all of them would be fine, and people wouldn’t be confused using any of them for Estela (and while I love both spellings, Estela is my favorite – I just think using the one L looks more exotic and different, like Gisele or Gisela vs. Giselle, Daniele / Daniela vs. Danielle, etc). My favorite is Esta, but Stella is cute too (maybe if you want to go with that, Estella would be the easier spelling). I also like the sound of Steli (or Stellie, Stelli, etc). I know someone who uses that.