Given that making claims about other (possibly dead) feeds has abuse potential, and is more complicated, I'd kinda lean towards just implementing the multidevice bit, for now, not trying to hit subjective merging with the same stone.

Okay, fair enough. I will leave the user interface showing subjective merges for now. This is definitely a lot less important than merging two-way linked accounts.

The reason I was excited about the prospects of subjective merging was that as a uxer, it de-emphasises the importance of backing up your feed, and worrying about complicated crypto stuff. Instead it becomes more like relationships in meatspace where you don't need an absolute ID to connect with a person. The "person" is entirely in the heads of your friends!