Just because we can't prove when Naruto became Hokage doesn't mean we have to automatically assume he just recently became Hokage.
This is exactly what I am saying, it's ambiguous, we don't know when he became Hokage but the recent chapter suggests it was way before chapter 700.
The only thing right here with what you said, is that we never saw Naruto become Hokage in the manga. But with the story Kishimoto has, he probably won't. It should be clear as day Endles, that if Naruto isn't the Hoakge in the Last, but is the Hokage in Chapter 700. It happened between those two time frames.
The chapter shows recently, I never stated it was that day exactly.
Now for the counter argument.
Sarada does not remember Sasuke, characters mention that he left before she could form memories. She asks him what was he doing all these years and why didn't he visit, then we get that flash back of the meeting, clearly implying that Naruto was Hokage when Sarada was too young to remember, or even before she was born.
Naruto himself asks Sarada to explain on what she knows about Sasuke, and when she says that won't happen he looks at her confused along with Chou Chou. Then he remembers he had been away for almost the entire time.
The reason why Sarada doesn't remember Sasuke, was because he wasn't there. This is not my including her as a baby, because we don't even know that far. However, we do know from the flash back that Sasuke's conversation implies the time frame which I explained way too many times, *and it almost seems as if this mission was brought up from a careful observation and study. Meaning it took a while for Sasuke to get there, that also seems to be implied when Sasuke gives his verdict on the hypothesis.
^This paragraph is speculation starting from the *