Absolutely it can stand on it's own! I love Tessa. I don't really go for 2 middle names, but this is a great combo!
Tessa seems kind of flimsy to me compared to your other girls. I don't think it has the depth and history for me to be enthusiastic about it to a any great degree. However, I do love Tess for its Thomas Hardy literary connection but Tessa lacks something (backbone?). It reminds me of Gemma and Jenna. The names are nice enough but there's really no substance behind them. I prefer some of your other options like Hannah.
AMAZING! It sounds so joyful and floaty but could easily work for a tomboy by being Tess.
Tessa Felicity Jane is pretty. I like Tess Felicity Jane more, but Tessa is sweet. I like both Tess and Tessa more than the fuller Theresa, but the "A" on the end just seems so superfluous, and it doesn't really make sense for me to use a nn, just to give my daughter with a nicknamey name another nickname (Tess). So I've gone straight to what I love the best--Tess. I do think Tessa is usable on its own, though, and if you like it better than just Tess, I think it's beautiful. I don't know if you remember, but Tess Felicity was my Tess combo before I came across Tess Penelope, which is my current combo. :)
:) thank you all so much for your thoughts and input!