"Well, because his arrows caught up to each other. That means his first arrow has to be shot so slow that the last can catch up with it (if he done so on purpose or out of some other reason I don't know). That means, whatever his usual arrow speed is, the first arrow needs to be massively slower than that. Under that circumstances I don't believe that the assumption for the first arrow to be as fast as a bullet is necessarily still true."
the speed of his shooting would account for that seemingly lack of difference, which by the way doesn't really exist if you look closely... also the same attack was shown to have every arrow at bullet speed, it might be said that the slowest one is bullet speed and the others are faster...but not the other way around...
"The second assumption is that all 1200 arrows were shot before the first arrow reaches the impact point.
In the scan we can see that there already is smoke at the impact point and we don't really see if anything has already reached it. So how do we actually now that some, if not most arrows haven't already impacted? If for example the first 1100 already hit, even with the first assumption the speed would likely not be notable anymore, right?"
that is a matter of opinion rather anything you can say is assumed wrongly, i can say the first arrows are only just hitting as we see them, and you can say that an indefinite amount of them have already hit. both assumptions are equally likely to be true due to lack of evidence, however mine makes the feat calculatable, and yours makes it uncalculateable, ergo my assumption is necessary for the calculation, it can't be done otherwise...
"Lastly one would also have to think about if the speed with which an archer, who has trained basically nothing else in his hole life, draws his bow would even scale to anything else in the verse."
i believe that those who devote decades to high speed movment are scalable to those who have devoted 8-10 years (at the most, as part of it was mixed with school life) to their marksmanship...