Morty followed the death stones directions the the letter. In most shows, there would have been some sentimental scene where he rejected this 'ends justify the means' mentality in favour doing what's right in the immediate present. That's not what happened, at all. Rick just ripped the stone off his head.

This means one of two things. Either Rick broke that timeline, or his actions are part of it.

So, how is even Rick capable of doing this? Is the future with Jennifer a definite do to these actions, or barely even a possibility?

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Rick was too quick. The death stone did show Morty his future death, but it was up to Morty to figure out what specific actions in what specific sequences led to that death. He had time to figure out what to say or do and prepare when dealing with everyone else besides Rick. Morty only knew his future changed because of what was immediately happening, and do to literally every other character not knowing what was happening and how Morty was doing what he was doing, Morty had time to adapt. With Rick physically there, Morty didn’t have a snowball’s chance in hell in figuring out what to do before Rick got to him. Morty is only human, and Rick is the smartest person in the multiverse (it didn’t help Morty that there were multiple Ricks acting against him).