I have had the same problem with dogs and the e-collars. I didn't find a good solution for them. But, my cat Mozart recently had paw surgery. When I removed the bandage he started licking the site and he wouldn't stop. I tried distracting him with movement and cat toys and that didn't work. What finally did the trick was offering him some Pounce cat treats. I had to give him about 5 or 6, but he stopped licking to eat them and afterwards he didn't go back to licking his paw.
If you have treats or a can of wet food or tuna, maybe that will distract for the time being.
The worse that could happen from licking: If there are disolvable stitches they could disolve to quickly. There is also a risk of infection to the site. If you choose to leave the helmet off, watch for changes to the site, such as redness, pussing, or fever that could indicate an infection. (and if these signs are there, take them back to the vet)
If you have to leave the helmet on, they'll live. Just make every effort to distract them from the discomfort. (Better temporary
discomfort, than an infection that requires antiboitics and lengthens their recovery time.)
Let me know if the treats work.
And maybe somebody else has a useful suggestion.