This is an excellent question, but the answer in some ways comes down to the definition of the word "god." To a certain extent, both religions began as an assertion that "this one god" deserves exclusive worship above all others. The Egyptian pantheon had many powerful supernatural beings. The Aten/Aton hiatus represented a unique period in which the worship of other entities was shunned in preference for this one. By returning to the old ways - the obedience to many deities - we can clearly see that the short-lived preoccupation with Aten/Aton was nothing more than a brief flirtation with henotheism.

Judaism presents a more complex situation because it is long-lived and even in the early period the belief system no doubt changed. Indeed, it is important to remember that belief systems regular change, and religions - even when "fixed in place" by a written text - typically still exhibit change over time and place.

That said, there are hints at the idea that at its start Judaism recognized that there were other gods; again, the assertion was that the "One God", was the only one worth worshipping and all other should be deprived of any hint of devotion. That would pretty much qualify that early period as fulfilling the definition of henotheism. But Judaism evolved, and it apparently became more monumentally monotheistic over time.

Of course, we must also recognized that most belief systems include many powerful supernatural beings. When historians look at pantheons in the past - or when early ethnographers looked at third-world belief systems - there was a tendency to use the term "god" promiscuously, applying it to all sorts of powerful supernatural beings. This is not always appropriate, and the dividing line between "powerful supernatural being but not really a god" and "powerful supernatural being that is one of the gods" can be extremely difficult to define.

When a nineteenth-century Northern European put out a dish of milk for the "fair folk" - the fairies/elves/huldrefolk/etc. (depending on the place) - was this a matter of worshiping a god? Few would suggest as much. But how different was that from an act of devotion directed at a "god" in pre-conversion times.

Either many more societies are henotheistic because most monotheistic religions include powerful supernatural beings other than "God" who deserve respect, OR we need to rethink the definition of what is a god. Most classical-era "gods" are not omnipotent or omniscient, and yet there was often a prime mover who seemed to have powers greater than the rest. But most "gods" are simply powerful. We cannot look at so-called "polytheistic religions" of former times and say that they had many gods when they are in some ways very similar to so-called "monotheistic religions" with one god surrounded by angels, saints, a virgin mother, ghosts, elves, mermaids, and a cadre of powerful demons led by a really powerful nearly-god-like Satan (many of which we would regard as "gods" if they appeared in a classical pantheon).

I hope that helps. I'm afraid I can't take you up on your kind and generous offer since I am otherwise committed. Pity really: while I'm a horrible cook, I have always been very willing to do the dishes!