You can use the .replace() JavaScript method to replace the 0 with 84. The expression might look like something similar to this: {{$json["number"].replace('0','84')}}.
For Case 2, can you please share how does the data structure look like? Are the numbers returned as a single item, or are they returned as multiple different items?
In the Format Number node (Set node) toggle Keep Only Set to false. This will return the data from the previous node as well. When it is set to false it will return firstname, mobile, updated, as well as the formatted number.