Thanks to Usagi I've discerned that there are many different kinds of suicide. Each with its own social requirements.
Seppuku is used when honour must be maintained.
There is no dishonour is being married. It may suck. But it's not dishonourable.
If she killed herself it may be considered "shinju" or lovers' suicide. And she would probably hang herself
And I don't believe Tomoe would kill herself. She is a samurai and would obey her lord if he said she was to marry.
And it wouldn't be that horrible. Think of her children, how blessed they would be to have such a mother as her.
The story possibilities are wide open.