Help and opinions about difficult PhD choice
Some general points I will address.


It is much easier to get a PhD in one topic, obtain a faculty position somewhere, then pursue a new niche field of research, than it is to obtain a PhD in the niche field in the first place. I did a dissertation on a topic I am not passionate about. But I was passionate enough about it that I could spend four years thinking about it then move on. In my current position, I do not do any research on my dissertation topic. I have shifted towards something that interests me more. All of this was facilitated by the fact that I already have a PhD. I can control my funding and research much more now that I have a PhD. 
This is based just on anecdotal evidence, but I have found that students who want to find a topic they are extremely passionate about usually end up unfunded and take several additional years to complete their degree. And often, in the end, they end up writing a dissertation on a topic they are not mind blowingly passionate about. 
If it is stability you are looking for, topic B may be where you need to look. You will need to decide between pragmatism or passion. 
Be aware that if a field is extremely niche right now, it likely still will be in ten years. This means that funding and opportunities will be harder to come by if you pursue faculty positions after getting a PhD. Unless it just happens that somehow your topic all of the sudden becomes in massive high demand, niche subjects tend to sort of stay just that: niche. 
I cannot give you relationship advice. But if you are going to be in a relationship, her feelings do need to be taken into account.