Wow! You just described exactly what we've been going through for the past couple of months. Our daughter (who will be 4 in May) has had a night light on for a while, that recently graduated to a bedside lamp (and looking at books -- even got a little clip-on book light). And now our son (22months), who of course wants to do everything his sister does, is sleeping with a bedside lamp on....
I soon realized where that saying came from: "if you give 'em an inch, they'll take a mile" or something like that.
While I do believe that she was a little scared of the dark, she has used it to manipulate us. Not that she's being intentionally malicious. It's a typical preschool control battle and testing her boundaries -- she realized that she had control of a situation, the boundaries kept moving ("ok, honey, you can turn on a lamp in addition to your night light" then "ok, honey, let's do something else to appease you.....") so she kept pushing and pushing.
The problem was that she became more unsettled the more we moved and tweaked and wiggled the boundaries.
With her constantly coming out of her room, and bed-time becoming an hour long process (or more), and we were getting stressed out about it, we finally decided to just be firm and set a limit. We've been doing this about a month:
First, we eliminate all excuses -- we read a book with her, she goes potty, and she gets a cup of water on her night stand before we actually put her to bed.
Then, she can go to bed with all these privileges:
her door wide open (this helped a LOT),
the night light and bedside lamp on,
and the hall or bathroom light on (she chooses which one of those -- either helps to brighten her room a little).
However, every time she comes out of her room, she loses one of those lights. She REALLY wants all of them on, so any time we enforce the rule, she ends up staying in her bed with rare exception.
There's often crying/screaming involved when we have to start switching lights off. Usually we let her earn the light back when she calms down and stays in her bed. She almost always earns the light back and we usually don't have to go through the "enforcement - re-earn privilege" process more than once in a night. When she loses a light, we're very empathetic ("I'm so sad for you that it has to be this way. I hope you can make better choices tomorrow and get to keep your lights on").
If we've had a bad night where she lost one or more of her lights for the whole night, we talk about it the next night to remind her and prepare her: "Are you going to stay in your bed after mommy and daddy tuck you in?" and "What happens when you come out of your room?" That usually does it (for a while, at least. I certainly don't think it's the end of it! We expect to re-visit this several times before it passes)
Also, I used to turn off all the lights after she falls asleep, but she'd wake up in the middle of the night crying and scared. This wasn't working for anyone in the household, but all the lights, while still dimmer than her overhead light, will keep her from having the deep, restful sleep she needs to stay happy all day (a period of light sleep around 3am is a normal part of her sleep cycle -- normally she'd drift back to being fully asleep, but the darkness made her scared and wide awake).
So, we came up with a plan and let her know that we didn't want her to be scared, but we also wanted to be sure she got good sleep so she wouldn't be grumpy. We asked "Would you rather be grumpy or happy? Would you like mommy and daddy to be grumpy or happy?" Even though she said SHE would rather be grumpy (which was simply her trying to be defiant :-)) she said she would rather US be happy. If she says she'd like everyone to be grumpy, you can have a little giggle about it and simply say "Well, we really like it when everyone is happy, because we can have more fun. You can even continue the questioning with "Do you want fun times, or not fun times?" and tie it all back to "Well, one way to have fun times, is for everyone to be happy instead of grumpy, and we are all much happier if we have lights off after we fall asleep. We have a plan that can help make all of our family happy so we can have fun times together."
So, our plan is that after she's asleep, the only light that stays on in her room is the night light, her door stays wide open, and we keep the hall light on, just in case she wakes up and needs to come get a mommy or daddy hug and get back in her own bed.
We were a little concerned about the part about needing a hug in the middle of the night, but it wasn't all that bad. Simply having the option to come get us in the middle of the night is reassurance enough. She doesn't actually follow through with that part very often.
With all this typing, I'm sure it sounds more elaborate than it really is. Carrying it out really isn't all that complicated. You just set it up, get her on board with it (even if she protests at first, just say "this is our family's plan" and stay the course) and be prepared to enforce it (with empathy, of course).
It's working for us. We just remind ourselves that she'll out-grow this phase. Of course, she'll have a new issue to replace this one by then...