Inconsistent application of draw borders script - google-apps-script

Question

I have a script scheduled every evening to look at a number of different sheets and draw borders around any new data entries.
The script works yet is inconsistent. When I check it works completely for some google sheets and partially for others. There are about 10 internal sheets on each google spreadsheet and the borders are drawn in for some and not for others.
I don't understand why it is not running clean.
function addBorders() {
//Access the sheet which contains all the IDs
var idSheet = SpreadsheetApp.openById('example');
//Check for the last row
var lastRow = 0;
var currRow = 2;
//Loop through the sheet and find the last ID available
while (idSheet.getRange("A" + currRow).getValue() != "")
{
currRow = currRow + 1;
}
//Set the last row with an ID found to be the lastRow
lastRow = currRow;
idSheet.getRange("B1").setValue("Total Rows");
idSheet.getRange("B2").setValue(lastRow);
//Loop through the ID sheet to find the ID to make changes to
for( var y = 2; y < lastRow; y++)
{
var studentID = idSheet.getSheetByName("Sheet1").getRange(y, 1).getValue();
//As IDs are take from the ID sheet, open the relevant sheet and run the code below
var ss = SpreadsheetApp.openById(studentID);
var sheetsCount = ss.getNumSheets();
var sheets = ss.getSheets();
//For each sheet in the individal student spreadsheets, set the borders correctly
for (var i = 0; i < sheetsCount; i++)
{
var sheet = sheets[i];
var range = sheet.getRange(6, 3, 35);
var values = range.getValues().map(function(d){ return d[0] });
//clear previous border
var selection = sheet.getRange(6,2,35,5)
selection.setBorder(false,false,false,false,false,false);
//set border
var index = values.indexOf("");
var border = sheet.getRange(5, 2, index+1, 5);
border.setBorder(true, true, true, true, true, true);
}
}
}

Answer 1

This is incorrect while (idSheet.getRange("A" + currRow).getValue() != "") you must specify a sheet (i.e. tab) example: idSheet.getRange( "Sheet1!A" + currRow).getValue()!="") If this works then it's probably always using ss.getSheets()[0] the most left sheet.
This will result in not accessing the data in the last row
for( var y = 2; y < lastRow; y++)
How do you know that the index below will never be -1. If it is then index + 1 = 0 which is not a legal row value. Seems a little flaky to me.
var index = values.indexOf("");
var border = sheet.getRange(5, 2, index+1, 5);
Spreadsheet.getRange