Well, if that happened in fanfiction, it would likely create a major rift within the superhero group. It could also lead to some intense action sequences and emotional moments as Tony tries to defend himself and understand Bucky's motives.
Well, it depends on the specific comic series. In certain story arcs, Bucky is shown to have a role in that tragic event, but it's not a constant across all comics.
Comic storylines are fluid. Thanos and Tony Stark's interactions vary. So, there's no standard way Thanos kills Stark. It could involve cosmic powers, strategic battles, or unexpected plot twists.
The reason behind Captain America killing Tony Stark in the comics was a culmination of various factors such as ideological differences, personal vendettas, and external manipulations that pushed them to this extreme point.