Oh no, I don't think I could do it either, I would think I would be jinxing the wedding if I did that. But I really don't think they are going to ask how well your witness knows the both of you, just the fact that your witness has known you both for over 6 months is what they require and you said she does, so you have nothing to worry about. Why they still have that archaic law, I don't know!
Haha Stress! Oh god yes, oh how I do know that feeling! I was doing the same thing and I had barely started planning our wedding- That's what made us decide to go back to the original plan of eloping to a castle in Scotland, except with a twist, having a marriage blessing in London and one in New York with our families and a meal after, I feel like a huge, gigantic, enormous weight has been lifted off my shoulders since we made that decision!
You really should try not and worry about the little things though, you will drive yourself crazy, try and enjoy this time as it gets closer to the wedding, you seem to have everything well organized and you have to remember there really is only so much that you can do! Your wedding will be perfect and everything you want it to be, after all you are getting married to your love, that's really what its all about. So try and relax and not sweat the small stuff. All the little details will fall into place and you will have a wonderful time celebrating your marriage with your new husband and friends and family.