I had some difficulty starting to answer these questions because both of them will strike me to my core so badly. It will feel like a huge part of psyche has been chunked away irrevocably, that I have been scourged by destiny once more, and I have no choice but to move on, as I will be forced to do so. If I will still move on.
The first one is because, one, Marisa has an interest in being immortal. If she dies, not only did she not achieve her dream, but also because I witnessed it. For everything she did to get to the top tiers of power, even her could not stop death even if she theoretically could. And I can only watch it, and not even be given a chance to anything, just anything, to save her. I might be able to continue a relationship with her, but I can only consider myself a widower. And whatever dreams I promised to her, it will be too late, but I can only do them now as a revenge against fate or destiny, or whatever force that will drive ZUN to kill her off like that.
The second one is basically harder on me. And for a very good reason, other than the obvious: Perhaps that will be the punishment that was meant for me after I basically cheated on her once. I stopped doing so and said sorry to her, but I felt like it was not enough to make it up to her. I know it is meant for me, but I have no choice but to deal with it.. As for the prospect of continuing the relationship with her, I would very much like to, even if I have to swallow my ego for it, but I will then become a literal wittol. I don't want to become a wittol, because I have a hard time swallowing my ego, as it is a great insult against me, and it is unacceptable in today's political climate. What's worse is that this is the most plausible scenario, canonically speaking: Rinnosuke Morichika, the sole proprietor of Kourindou, is a childhood friend of hers. But he's more of a brother figure to her, so I can only hope and pray that ZUN will not delve into this scenario, and that the Westermarck effect will stop it.
But my waifu is Marisa Kirisame from Touhou Project, and the nature of the series itself makes tackling this kind of topic deeply and seriously a pretty large stretch in the first place, as it has been said by Meilingbro and Flandrebro. Still, that fact does make the both prospects worse, because it will be a rude awakening to us all.