The first question is around a premature ending of the tefila by mistake, there are several problems, the fact that he already leave the tefila, and if we understand that he did not leave, the movement backward is perhaps an interruption in the middle of the tefila.
Here the movement itself is not wholy intentional from one point of view: He did not want interrupt the tefila before the end. But was convinced that the tefila is over. May be that to intend to leave the tefila is a stronger problem than interruption with intend to return to tefila.
One case has some similitude in Shulchan Aruch Orach Chayim 422, 1: A man was praying Shacharit or Mincha on Rosh Chodesh, but he forgot to say "Yaale Veyavo". Now, he suddenly becomes aware of his omission. But he already stepped back from his place. The Shulchan Aruch says that he must go back to the starting point of the Tefila. Mishna Berura adds than if he still not stepped back but already finish the Tefila, the rule is the same.
What may be learn from this case? He has not yet fulfilled his duty, but the Tefila is stopped (almost equivalent to interrupted). He needs to start from the starting point.
Additionally, in this case, the end was the result of an error.
he mistakenly thought he was up to עשה שלום... for a second
--> He needs to start from the starting of the Shmona Esre.
The second question:
Shulchan Aruch Orach Chayim 104, addresses moving from his place at the time of Tefila as an interruption (most cases of interruption covered in halacha are interruption by speaking). We find there several cases of moving, some interruptions are allowed and are not regarded as great interruption, some else are allowed but are despite this great interruptions (e.g. because they are too length). Some movements are not allowed.
because someone bumped into him
It is as an authorized (because he had no choice) and short interruption, e.g. this case.
--> He must return to his place and starts from the begining of the Beracha in which he was SA OC 104, 5