Hi,
This is actually my first post on this site. I'm a 22 year old female university student who has lived with nocturnal enuresis my entire life - persistent nocturnal enuresis, I believe is the term.
As disappointing as it may be, there may not be a solution yet. There wasn't for me.
My parents were well-intentioned - they tried everything for me, alarms, pills, doctors, testing, restrictions, and nothing ever took. Looking back on it now, I'm actually quite bitter about how it impacted me. I had to stop drinking at 6 or 7pm, would wake up in a panic too late for the alarm, never went to sleepovers... It was traumatic.
They and my (ill-informed, disinterested) doctor were specifically against overnight protection - the idea was that I wouldn't learn to be responsible. In reality, that is often the best choice.
Truthfully, given that she is 16, it is unbelievably unlikely to resolve itself for her. The stats just aren't in her favor. She's going to have a hard time for a while yet. You have to work around your NE and that can be frustrating and difficult.
From your post, it seems like a lot of the diagnostic tests have been done already. Typically, adult and teen NE isn't because of a physical problem, as it's often found earlier. It's not impossible, but unlikely.
Part of the challenge for me at the same age was accepting what I have and learning to live with it. I don't let it stop me. I'm nearly always dry, save a few random nights, a lot like your daughter, and my NE has actually improved over the years. I lived in my university residence for a few years without too much worry. I bought a very good waterproof mattress pad and found myself okay nearly every time. If I stay somewhere else, I use overnight protection. Life isn't going to end for her, and it can be made to barely have an impact on your daughter. One of my fears was having a boyfriend and what would happen, but I have one now, and while we haven't discussed it fully, he knows. It didn't stop us.
I know and completely understand what you want to do for your daughter. You want to solve the problem she's facing - you want to help because she's your kid and you want the best for her. Ask her what she wants to do. The reaction to the "you'll grow out of it" rings familiar to me, and all I wanted was to end the constant work I was put through. She may want to keep going, but it's valuable for her to make that decision and to do her own research as well as you. It helped me feel like it wasn't my fault and that I wasn't the only one - in fact, it helped me see that I still had a future and NE wasn't going to ruin my life.
Good luck to you, I hope for the best for you and your daughter.