How to get .JSON file data as source data to Power Apps? ( .JSON file is in share point )

As you have identified your problem is the writeback.  Having a JSON file as a data source is all nice, but writing into it is much harder. There is a JSON function in PowerApps that you can use

JSON function - Power Apps | Microsoft Docs

but it basically means that you need to ingest the entire JSON file when you start the app, store it (in a collection for example) and then write the entire JSON file back to the storage space when you make changes. 

That will likely prevent concurrent use of your app.