LEDs, Reverse Polarity, and Battery Life
My opinion is that it is safe, and all single LED's use a -5V Vr max spec.

But for leakage, there may be exceptions beyond your requirements due to wounded LED's that leak more than spec, from overstress due to static discharge, solder temperature, operating temperature, moisture ingress and others, which all affect leakage.  Also increased chip sizes from 0.1W to 1W to 10W will undoubtedly leak more from total area, but vary from OEM to OEM on specs.  I can't speak for all Chinese factories of flashlights to see if they buy LED's that are guaranteed low leakage, 1vs 10 vs 100uA but low leakage is a quality factor to consider qualifying a source of LED's in large volume for reliability reasons. As aging factors, contamination, moisture ingress all affect leakage as well as ESD/EOS.

Using a 1MOhm series resistor you can create a curve-tracer using a scope, signal gen. and xy mode and compare LED leakage specs, if you have a batch and verify as I have done on a sample basis. Solder temp over stress can also affect leakage.

In the end your lithium coin cell leakage when cold may be good for 10 years and when hot maybe only a few years. The reverse connection idea may be adquate, but the flashlight designs I have seen all include a screw cap threaded switch contact or momentary if it is a multi-function flashlight, then use a momentary switch. Then they use a plastic ABS shim in series with the battery with a pull-away tab or removal instructions for shipment to avoid power activation during shipment.