Well, Here we go again
"The only part that has ever been about "safety" in the entire chapter was the resilient phone thing she suggested at the start that would not break even in battle"
And the GPS part. And you ignore that the reason why Momo looked for the most efficient phone was because she wanted to keep Shamiko safe.
"But that was just her first instinct based on Shamiko's specification (aka makes calls in emergencies)."
No. https://mangadex.org/chapter/788455/2 , https://mangadex.org/chapter/788455/3 Shamiko told Momo from the beginning what phone she wanted, but she said "No, you should choose this one."
"She discarded that idea right away after Shamiko told her what she liked."
As I said before, She listened to Shamiko in the end because she cared what she wanted. But first she think about what would be better to keep Shamiko safe.
"The one at fault in this situation is clearly Shamiko. Momo's obliviousness is just another facette that gets explored"
I disagree. Clearly, the both were wrong in this chapter. Momo must have listened since the beginning what Shamiko want (and be more attentive to her feelings), and Shamiko must have been more honest. It was the fault of the both.