Regarding why mc is resistant to use the walkie talkie, Its pretty much spelled out in the bus storyline and heist storyline.

In the bus story line the original timeline without walkie talkie would have ended with the past victims. Through the walkie talkie, the mcs inadvertently caused death of the bus attendant (present) as well as the innocent jogger who suffered a stroke or whatever it was. They effectively traded one live for two.

In the heist line, it should be safe to assume the bridge would have collapsed anyways, but, it is unclear if the events would have turned out the way it did without the mcs interferences. Remember that young mc questioned the old mc about the heist (unsolved at the time), which arguably prompted old mc to chase down the leads and wrongly arrested the thief. Possibly lead to death of the thief's daughter. This set off the chain of events in the present where the saved girl is kidnapped and female mc gets blown up. Even though they did find the guy responsible in the end, no justice was served and the thief killed someone else (employee of bridge building company) anyways. By chasing down this unsolved case the mcs possibly caused death of the thief's daughter and the bridge employee, ruining the thief's live in the process.

At this point it should also be apparent that there is no way for (young)mc to know about anything. He can't be sure that the outcome is changed for the better or worse - thief and his daughter didn't make the news from what it seems and likely lived happily ever after. He couldn't have predicted the murderous spree of the thief even when they did clear his name. The point is, like the Ashton Kutcher movie Butterfly Effect (or Frequency or any other similar movie) , that there's no way to predict what will happen if you change the past. The young mc is torn between uncertainty and desire to see justice served.