That's an excellent letter, and I hope your visit goes well.
I've been debating the merits of the letter vs. phonecall vs. in-person coming out talk with my parents. I'll be interested to hear how this approach works for you.
If it doesn't go well I certainly wouldn't attribute it to how you worded the letter though. That seems quite good. Now you just have the followup questions to handle. I hope they go well for you.