How to reject changes to non-null cells generated by a query - google-apps-script

Question

A sheet populated with results of a query, from a separate workbook. User can select values from a drop down for any cell. When onEdit called, value is transferred to the main workbook, which then replicates back to the current cell in the active sheet via the query. Trying to prevent cells with existing values from being edited - ie - user can only put a value in a currently blank cell, and not change an existing. Because the query repopulates with each change in the master, locations of empty cells constantly change as they are filled or if new rows are added in the master.
(code below is a mashup from this video and one similar about unique ID's..(https://www.youtube.com/watch?v=ucLO4iHP2Kw))
Tried reading the current range of data into an array, and chugging through to set protection on any non-null cell. This works, but a 3x60 range of cells takes over 2 minutes to set protection on the non-blanks. Not a very good wait time before the user can move on to edit another cell!
Following is current code triggered on onEdit() The section after the "else" is where I want things to bail out by rejecting the user selection/input.
Any attempt to enter something different or change a value in a non-empty cell results in the query generating an error because it doesn't want to overwrite the cell. Hitting the "Del" key clears the cell, and the query then will refresh everything. Optimally, it would be great if the section after "else" just did the same - hit the "Del" key!
function syncData(e){
var src = e.source.getActiveSheet();
var r = e.range;
if (e.oldValue == null){
r.clear();
let id = src.getRange(r.rowStart,1).getValue();
var db = SpreadsheetApp.openById("SPREADSHEET ID").getSheetByName("SHEET NAME");
var ids = db.getRange("A:A").getValues();
let row = 0;
for (row; row < ids.length; row++){
if (ids[row][0] === id)
break;
}
row++;
db.getRange(row,r.columnStart).setValue(e.value);
}
else{
e.setValue(e.oldValue);
}
}

Answer 1

Of course I came to a solution after posting here!
If the rest worked, all I needed to do was post e.oldValue to the main spreadsheet, rather than e.value. That would repopulate the view sheet with the original value, preventing the user from making a change.
Here's the modified code:
function NewsyncData(e){
var src = e.source.getActiveSheet();
var r = e.range;
r.clear();
let id = src.getRange(r.rowStart,1).getValue();
var db = SpreadsheetApp.openById("SPREADSHEET ID").getSheetByName("SHEET NAMES");
var ids = db.getRange("A:A").getValues();
let row = 0;
for (row; row < ids.length; row++){
if (ids[row][0] === id)
break;
}
row++;
if (e.oldValue == null){
db.getRange(row,r.columnStart).setValue(e.value);
}
else{
db.getRange(row,r.columnStart).setValue(e.oldValue);
}
}