I don't have any answers, I am more n your shoes. Dating a guy who lost he love of his life. The only thing that brings me comfort is that when I had each child, I did not have to stop loving the previous one to love the next. There is room for all of it. Enough love for everyone to be loved in their own right. Just because BF loves and misses his previous GF (he never got the chance to propose. By the time he got to her she was on life support.) doesn't mean he will not be capable of loving me (his commitment issues seem to be coming from somewhere else entirely) fully. That is what I tell myself anyway. I expect she will always hold a place in his heat, that it will always be reserved for her. but that does not mean that he cannot love me. Weather or not he will is another story but I don't blame his love for her or hold that love against him. Does that make sense?
The truest answer to violence is love. The truest answer to death is life. The only prevention for violence is for the heart to have no violence within it. We cannot prevent evil through any system devised by mankind. But we can grapple with evil and defeat it, but only with love—real love.
It took me a while to accept my first husbands death and that loss has made me so thankful both for the time I had with him and for every day I have with my husband now.
I'm not sure how to answer your questions. I love that my husband is not threatened by my love for my first husband. He knows yesterday was a hard day for me and offered a hug and acknowledged the anniversary. I don't think there is much you can do except be there for all of them including your fiancee. You can encourage counseling if that's helpful and listen.
You will never replace his wife. That pain will make him who he becomes and never leave totally. That doesn't diminish how he feels about you or make him love you less.
I'm sorry for his loss and the childrens' loss and wish you all the best as you travel this journey together.