Here's YOUR biggest problem - that you can't see a way out for her. There are thousands of ex JWs have found it but it is very difficult. It can take a long time. Still, you need to be positive so you can help her.
Its natural for you to find it hard to deal with serious problems like this but it's easy to see that you want to do what's best for your daughter.
You are in a tricky situation yourself as you need to be careful how you show your support or there will be reprecussions for you also, both from your wife and fellow JWs. You need to consider how far you are prepared to go and what your priorities are.
The best way to take this is one step at a time and you've already taken a good course.
You support her in stopping the door to door work and you hav made sure she is seeing a therapist. Since she is seeing a therapist as you recognise she is depressed then now might be the time for her to stop going to meetings (midweek first) if that is what she wants to do. If she is questioned about this you can get the Elders to speak to you as you are the head of the family and explain that due to her health you feel she needs a break.
It's hard to get a handle on the whole story through the Internet and I do wonder how much her situation with dating a JW that she has decided she no longer wants to, plays into this? It's tough breaking off a relationship as a JW because it has reprecussions in the congregation. Just one aspect is the guilt feeling that you should only be dating with a view to marriage.
All this (the boyfriend) may be the root of the problem and once she can find her way of getting out of this without any scars she will be in a much better position to decide what steps she wants to take about the religion.
I am trying to say that you need to be careful so as not to add to her problems, if you encourage and push her or are negative about the religion at a time when this is not an issue with her you may end up regretting it.
It is such a difficult time for a young lady to deal with her own emotions and know what she wants.
You sound like a lovely father.