1) We do find examples of the foolishness of the Yetzer Hora. (This does not answer your philosophic point.)
One example is in his confusion with our blowing of the shofar in Elul, not on Erev Rosh HaShonoh and on Rosh HaShonoh itself as this essay shows, page 10 by Rabbi David Myer
“However, our Rabbis teach us that on Erev Rosh Hashanah one is not
allowed to blow the shofar. It seems very strange that at the end of
the month, one day before the peak of our shofar blowing, we suddenly
stop.
Stranger still is the reason given, as we are taught that the
intention is to confuse the satan (our evil inclination). We are
taught that the satan hears the shofar each morning and prepares for
battle on Rosh Hashanah, the Day of Judgement. However, on Erev Rosh
Hashanah he notices that we have not blown the shofar and thinks that
he has missed Rosh Hashanah. So the following day, Rosh Hashanah, we
hear the shofar and embark on the Ten Days of Repentance, and the
satan is caught unawares.
We have to wonder, how stupid is the satan? I can understand his making this mistake the first year, and being caught unawares on Rosh
Hashanah; but surely after a few years he has figured out the trick,
made a note in his diary and knows that we don’t blow shofar on Erev
Rosh Hashanah. Is it really that difficult to figure out? It is
perhaps this very aspect that our Rabbis wanted to highlight. The
satan is indeed stupid and can be overcome with such a simple and
straightforward trick.”
In The Enemy Within: Confronting Your Challenges in the 21st Century
Rabbi Twerski quotes Reb Yisroel of Ruzhin who explained the foolishness of the yetzer hora voluntarily accompanying Reb Yisroel when he was forced into prison.
2) On the other hand, Rashi on Koheles 4 (13) understands foolish in the sense that he makes fools (of us).
and foolish: for it misleads him in the way of evil. In this manner,
it is interpreted in the Midrash.
Rabbi Moshe Aharon Stern in his book “From a Pure Fire: A Series of Shmuessen”, p207, quotes the Chofetz Chaim (similar intent to Rashi) to say that every craftsman is called by the name of his product. So the Yetzer Hora who makes fools of us is called a fool. (!)
3) Similar to (1), I remember but cannot source the idea that the Yetzer Hora can be pushed aside with quite unsubstantial arguments to avoid doing a sin. For example, one can say “Yes. It would be pleasurable to do it. Let’s do it later.” This would be without a clear definition of when "later" is.
We may be able to infer this from the encounter of Yaakov with Esov Braishis 33 (14) where Yaakov tells Esov that he will meet with him in Seir but in fact delays until the time of Moshiach under very different circumstances.
