I think you are right about her hoping that by crying, you'll change your way of ending night-night time and cmoe back in. And I think you are right to not go back in.
Since she had a good bedtime routine before this, she is probably going through some changes in her development, which partly makes her test you at bedtime. It could also be a reason she is waking early, unless the issue stems from no longer needing an afternoon nap.
Stand firm in your expectation of her self-soothing without your coming back in the room. I don't think there's anything wrong with a nightlight - how about finding some in a store that you are willing to buy, then take her with you and show them to her. If she wants one, let her pick between the ones you would let her have. If she has even more trouble going down once you put it in her room, you can always tell her it has to go if she doesn't go to sleep.
It is good to get this handled now, before potty training starts. Then things might change again. A potty chair in her room at night so that she doesn't go outside her room might help you then. (Of course, unless she sleeps through the night and has wonderful dry nights! Oh, we can dream!!)
All my best,