Google-sheets – Restrict or prevent drawing functionality

A fairly readable way of stating the condition of cell r being in a range is

var range = s.getRange("C3:W26"); 
if (r.getRow() >= range.getRow() && r.getRow() <= range.getLastRow() && r.getColumn() >= range.getColumn() && r.getColumn() <= range.getLastColumn()) {
  // do something
}

One could also hardcode the row/column boundaries, but this is not as easy to modify later, especially if your range is something like U2:BB8.

When you attach a form to a spreadsheet, the following happens:

1. A "Form Responses" sheet gets created. Its top row has some columns bound to specific questions on the form.
2. Each time the form is submitted, a new row is inserted in the sheet, at the end of the current data range. The form input is placed in this row, in the form-bound columns only.

The Form Responses sheet is a normal sheet with just two restrictions: (1) a form-bound column cannot be deleted; (2) the header row cannot be deleted.

So, nothing stops you from inserting a new column C for your own use in between the form-designated columns. The data you enter in the sheet manually will never be overwritten by the form, because the form input always goes into a new row inserted in the sheet. Feel free to reorder the rows, reorder the columns, delete any rows you don't want, add your own rows, add your own columns and fill them with whatever data.