>>279951118
Having worldlines that are defined and differentiated by key events is not illogical or contrived.
>sometimes things cannot ever change,
Anything CAN change, some things just would require a shift to a different world line.

There is no "problem" with the time travel rules, and more importantly that's just the plot framework to hang this story on. Steins;Gate is not about time travel, it's about fate and the relationships between these characters.