Hi C.,
First of all, congratulations on your engagement!! It sounds like you have found a wonderful man with a loving extended family who has embraced you; what a gift!
I am actually not writing you as someone who has married a widow, but as the child of a widow who remarried. When my mother remarried after my father had passed away, I was really happy that she had found a wonderful companion who she could share her life with. My stepfather had been divorced from his first wife, and I watched him go through many of the same feelings and concerns that you are sharing here, and I thought my insights might be helpful for you.
It is fortunate for your husband-to-be that you have also loved people who have passed away; more than someone who hasn't gone through that type of loss, you have that place of reference and can use it compassionately. You are right; while divorce has its own pain, losing a spouse by death is not the same as a divorce at all. Indeed, sometimes, it can be harder to recover from a divorce because of all the additional issues that come with that (betrayal, loss of trust, the unresolvedness of it, etc.).
As my mother has said, the love and marriage that she shared with my father was one chapter in her book of life, and her love and marriage to my stepfather is yet another chapter. Both are great chapters; one is the past, and the other is the present. While is is understandable to have feelings of jealousy, your husband loves you for you, and there's no reason to compare yourself to his first wife. There is no "other woman" and you can't "lose" him to her. He loves you -- and you know from his past marriage that he's a one-woman kind of guy.
My guess is that if you had met this woman, you two would be good friends. You probably have similar values: you both love and were drawn to the same kind of man. I imagine that if the 25-year ago version of this woman and your husband had moved into your neighborhood, you'd like them and welcome them.
Additionally, there are probably lots of things your future husband learned about being a great husband from being in a successful marriage for 30 years. He will bring these into his marriage to you. That is a gift you have, so to speak, from his first wife. So try not to look at this woman as an adversary, but as an ally. You don't need to compete with her; just be yourself, that is the woman your husband-to-be is in love with.
Which brings me to closeness and boundaries. I understand the need to set boundaries; they're a good thing. However, ask yourself what your motivation is for the boundaries you are setting. If they stem from jealousy or insecurity, it might be good to reexamine them, perhaps with a therapist.
The more your husband can feels he can share with you about his life before you -- which includes memories of his first wife -- the closer it will bring you two in your present marriage. Your being open and good listener about his past doesn't diminish your present life together, quite the opposite. It accents what a wonderful, loving, giving, solid partner your husband-to-be has found in you. There's tons of love to go around, and your being open about the love that existed in your husband's life before you met will only add to the love you two will create in your marriage together.
You have a unique opportunity to share not only in the future you have together, but to reap the rewards of the closeness and the good times he had with his first wife by encouraging your husband to share memories of those with you. Then, instead of him having a 'secret garden' walled away in his heart, reserved only for his feelings for his first wife, where he knows you do not want to enter, he will be able to share all of his heart with you, including the places she had touched. Then, truly, there will not be any place in his heart for comparison, because there won't be anywhere in his heart that you do not know, and love.
Your husband was blessed to find you and to have the opportunity to have a wonderful marriage with you. I wish you both every happiness.