I felt safe as a single woman traveling alone. The apartment was clean, quiet and spacious. The bed comfortable.
This part of Venice is a 20 or 30 minute walk to the total madness that the San Marco area has become. You can take a vaporetto (water taxi) if you want to save your feet.
I stayed in Sant'Elena because it was a short walk to the Venice Biennale. I found a few restaurants and a bar open sporadically in November and grocery store 15-20 minutes walk. There are a few very small groceries selling meat, cheese, bread, wine and some canned items right down the street. Osteria da Pampo was a good restaurant.