Even though it seems drastic, getting the lawyer ready is a good idea. It's understandable that you don't want out of the marriage but at least you aren't fooling yourself that something is DEFINITELY wrong.
As a last resort, I would sit her down one more time and force her to have a BLACK AND WHITE conversation. No bullshitting, or accusing anyone, or arguing. Just sit down with the following questions and CALMLY and RATIONALLY ask/answer them:
SHE should ask YOU:
1. Is there ANY other reason you took off the wedding ring other than the skin rash?
(obviously your answer is no, but she'll need to hear this one more time).
2. Have you ever cheated or thought of cheating on her or would you in the future?
(I'm assuming this would also be no...)
YOU ask HER:
1. Is there another reason besides me taking off my ring that you are contemplating out of this marriage? Make sure she knows this is her last chance to answer honestly.
2. Restate that you have and always will be faithful to her, and will work better to communicate with her if you have to take off your ring because these things happen and it's WRONG for her to go crazy. Make sure to tell her that a piece of jewlery is a symbol of your marriage but you taking it off to clear up a skin rash is in NO way violating your marriage. Then ask her "Do you believe I'm faithful and that this is just a misunderstanding, would you like to fix this and move on?"
Obviously that will be the final question...and if at any point in time she becomes irrational or beats around the bush or tries to complicate the question, just remind her that it's a BLACK AND WHITE conversation and you are trying to make some progress. Arguing and (potentially lying on her part or hiding a secret) aren't going to help solve anything.
I'm a female and I find it infuriating that you can't solve simple arguments with other females because they will argue over anything instead of just addressing the simple questions/problems at hand and thinking about their behavior/reactions in a rational manner. Honestly, if she can't be an adult and have this conversation with you and make progress in ANY direction...I would have second thoughts about her anyways.