I'm uncomfortable with characterising Tsumugi's behaviour as abusive, because it's clearly not malicious, and it's clearly not driven by trivial and selfish reasons - she's genuinely extremely uncomfortable with this whole situation and with how Amamiya feels, but she also cares very greatly about her and doesn't want to hurt her despite that discomfort, and she's doing her best to thread that needle.I really REALLY disagree with that.
Even if you don't reciprocate someone feeling, you can't just tell them to just STOP feeling things. The only reason her senpai is even going along with it is because she cares about Tsugumi more than her own well-being, but what Tsugumi is doing is pretty abusive. I'm sure she probably didn't intend for it to be this way, but she selfishly wants her relationship with her senpai to go back to what it was, no matter how much it would hurt her senpai to do so.
At the same time, Amamiya seems to have seriously underestimated how strongly Tsumugi feels, and seems in particular to be struggling to grasp what Tsumugi is feeling here. This chapter drives that home hard with the "I like you, I love you" scene - Amamiya seems to feel like she just has to make her love (and the joy she's feeling because of it) clear, and it'll be accepted; she's hit so hard by Tsumugi's response (page 13 really is a massive gut punch) because she's hit by the realisation that Tsumugi knows how she feels already, and that knowledge is only making things worse for her.
They've both found themselves in a place where attempting to protect themselves will do as much damage as attempting to protect the person they care about, and there's no way that either of them are at fault - the worst you can say is that they didn't communicate their feelings as well as they could have, but they really didn't even know how they were feeling until it was too late . . .
I think the key to all of this drama is that Tsumugi herself doesn't understand why the red string (and the whole idea of a "fated love") makes her so terribly uncomfortable. If this was something she could properly articulate, even just to herself, I think she and her senpai could work things out - Amamiya's deep and heartfelt joy at being able to say "I love you" to Tsumugi makes it obvious that her feelings are in no way tarnished by any thoughts of "fated love", and it seems like the only reason Tsumugi hasn't recognised her own feelings for Amamiya (which are at minimum a serious crush) is because whatever makes her so uncomfortable with the idea of "fated love" has left her afraid of the idea of any kind of love.
But as I said, neither of them even realised how they felt until they were in too deep for there to be an easy solution. It's honestly hard for me to see this ending without a tragedy tag (though hopefully on the milder end of things) . . .