$(1,1) = $(1,2) + $(1,3); // cell value ref: $(row, col)

  $('sheet1').onready(function() {
  }); // you get the picture

the spreadsheet programming and the data grid/presentation could be made separate.

replace fill handle based formula extension with parameterized formulae:

  for (int i = 0; i < $('name_range').length(); i++)
    $(1,i) = $(2,i) + $(3,i); // and so on and so forth

write as many functions as necessary.