Call me crazy but I think this is actually a sequel to the other behemoth series that the author is infamous for. I checked the webnovel raws for this adaptation, and the elf's name that the behemoth was remembering was called Aria, not Alice. Aria is the elf protagonist of the other series, and the visual description of her in the webnovel check out in the manga as well. His past memories only showed that he and elf girl went and defeated the demon lord, but he got killed in the process. Now we have this elf girl who's basically the spitting image of Aria, meaning Aria probably had children.
Unless this isn't some alternative timeline stuff or the author isn't gonna retcon stuff in the other series or this series, I have finally answered one of life's greatest questions...
THE ELF WILL NEVER GET THE CHANCE TO FUCK THE BEHEMOTH PET.
If she truly did and had children with the behemoth, it would mean that the behemoth of this manga is fucking his granddaughter, which is wild.