Well, it's fantasy, so there is no problem. If the author wants there to be a way for Shiori to die, there will be a way. I agree with your reading of the character's current intentions, but that doesn't mean she will actually do it when the time comes.I think the binding pledge is somewhat the easier of the problems because we still have enough arcs left where the author can throw in whatever new metaphysical rules that serve as dei ex machina. What needs to happen for that to make sense as a story is for it to be set up early and also, most importantly, for Hinako and Shiori to discover that they really want to live with each other.
Poly is probably way less likely when the Hinako's relationship with Shiori is portrayed as much more special, especially in contrast to the relationship with Miko. I wonder if they can remain close friends, though.
I think one problem is that we don't even know if Shiori is capable of dying at all. We know for sure a dynamite isn't enough and she may or may be able to will a death into existence. This chapter implies that she's rather ready to devour Hinako when the time comes and just cope with the loss for the remains of her existence(which honestly is even harsher).
Thinking about this, I feel like another ending could possibly be Shiori becoming mortal somehow in a sort of reverse "eating the mermaid's flesh". I have the impression that Shiori doesn't really like being an immortal, so that might be the happiest ending we can get.