It would be nice if a section were added on replacing a broken/burned out LED. As circuits using LEDs age, more and more people will find it necessary to replace them.
To be blunt you are over making this more complicated than it needs to be and over analyzing and over thinking the replacement, for all practical purposes...
You were told the basics of what to do in the first reply, just get an LED of the same size, color and type... The specifications of LEDs are not 'identical' across the board but in almost all cases they are plenty close enough for interchangeability as long as you match size, color and type (and color/size is not even all that important in many cases)... This variance in specifications is true with most electronic components and LEDs are not an exception, you rarely ever need 'exact' specification replacement components...
You said it was a "5mm LED with a red lens" I'm going to take that as being a red colored diffused translucent lens, meaning that it's a 'standard' LED not a super bright that almost always has a clear, tinted transparent or milky lens...
Like this?
If so it's just a 'standard' 5mm red LED, and there is a 99.99% probability that any 'standard' 5mm red LED you pick up will work just fine... Do mind the polarity though when installing, look for the flat side at the base of the LED...
Like I previously asked "Measure the voltage on the other side of the resistor (not the LED side), from that we can take a very educated guess at the LEDs perimeters..." the guess we could make with that value and the resistor value you have already given, will enable us to confirm the compatibility of a new LED without doubt...
The LED is there to alert someone that one of the Smartcells has suffered a failure and needs to be removed from the string. Failure to do so can result in the batteries overheating causing them to leak or explode, depending upon the failure mode.
As I suspect the LED is just a visual minder, it does nothing to actually 'protect' or 'provide' a line of defense, it just indicates a failure and leaves the rest up to you...