Oh dear what an unfotunate situation to be in! I can appreciate that you do not wish to intentionally upset your friend or make her feel alienated, but you need to remember that this is your day and the most important thing is your happiness. You should not have to be worrying about how one of your friends is feeling about the arrangements. You have tried to accommodate her in other ways and in my honest opinion doing a reading at your ceremony is a very generous alternative option. Clearly you have tried to discuss this with her and explain your feelings and reasoning and I assume she has told you hers. If your friend cannot accept this then the only option that I can see is to be firm and say that if she is not happy with your offer then perhaps it would be best if she were not involved at all. It may be easier if someone else could talk to her about this, someone who is not directly involved in the arrangements and could therefore give an unbiased viewpoint to your friend.