Have you thought that maybe she has anxiety about your relationship that is manifesting itself in a need to hold on to the past? You are talking about "the next big step" in your relationship, and that easily brings anxiety to the surface -- even subconsciously. There are several reasons for anxiety in this scenario; I will try to list few that come to mind.
Something is missing in your relationship that is making her romanticize a former "bad" relationship. Try improving your communication together on what your needs are. Listen to her. Something may seem insignificant to you but be a big deal to her, and vice versa.
She may be afraid of loss. Again, you two are taking another step in your relationship. What I gathered from your post, this man was her last "serious" relationship and he died. She may be afraid of losing you, not necessarily through death, and therefore is regressing.
She did not allow herself to fully or properly grieve over his death. She is talking about moving in with you, and perhaps this makes her feel guilty. Maybe she feels as though she is "dishonouring" him by moving on with you and therefore desires a permanent reminder of him.
These are just some thoughts I had while reading your post. I could be horribly wrong on all accounts. Communication between the two of you is the only thing that will help resolve this issue. Her body, her rules... but maybe this tattoo is not about the former fiancé, rather anxiety about her future with you. I wish you both luck!