a) Travel time. It sucks, I know, but you don’t “fix” travel time. It’s not an issue.
b) Latency. It sucks, I know. But unless you want to change how the entire game handles projectiles and hits (which introduces different problems), you don’t “fix” it.
You got the effect on your client, but your client didn’t have the time to send it to the server (or something like that). Not really “fixable”. You can change it, but then Tracer is gonna open a thread complaining about Kiriko surviving even though she used Suzu too late.