1. Trying to offer some rationalization of how the segulah works seems to me to be a waste of time. A segulah by definition is something whose workings cannot be explained. After all is said and done about the parsha of mon focusing our bitachon, or whatever other explanation is offered, it's still a mystery as to why this particular parsha on this particular day should have an effect.
2. Parnasa does not mean wealth. If anything, the parsha of mon teaches us to be satisfied with what we have. To borrow my wife's words on the topic (link):
Beyond that, though, the mon came down each day with just enough for that day except on erev Shabbos when the double portion came for the next day). That meant that the Jews in the midbar never had the feeling of security that comes from pas besalo [bread in the basket, on hand for later]. Each day was another challenge in bitachon that Hashem will provide what you need.
The way the term parnassah tends to be used today is not that you live day-to-day with bitachon that Hashem will provide but that you are comfortable -- secure in the knowledge that you have a number of baskets filled with bread and whatever else you want on hand. I get the sense that people consider the segulahs for parnassah to be guarantees of a certain standard of living that is quite different from the experience of eating mon.3. The popularization of segulos always brings out the segulah scrooges who think everything of this sort is illegitimate and feel it is their duty to convince the rest of Klal Yisrael of their point and make fun of the cretin neanderthals among us who would believe in this superstitious magic. I once gave a shiur and noticed that one of the participants reacted to every other line in the gemara with either a nod of approval or a question on the logic employed. My 2 cents is that when you sit in judgment of a sevara of Chazal, you sit in judgment of yourself. If you think Abaye or Rava's logic is faulty, it means your logic is faulty. If you think Abayei or Rava got it wrong, it means you got it wrong. Hillel and R' Akiva, Abeyei and Rava, Rashi and the Rambam's words define what the standard of Torah is -- there is no other external source of any sort against which they can be measured and judged. Everyone who is a frum Jew understands this. What some people seem to miss I guess is that R' Mendele m'Riminov is also a standard, a ruler against which other things are measured.
Now that I provided a great line for the segulah opponents to take out of context, a little clarification. Of course I don't mean that the opinions of a chassidishe Rebbe living not to long ago carry the same weight as statements of Abayei and Rava. To put what I mean in another context, I stand as much chance of jumping to the moon as jumping to Jupiter. Of course I know there is a difference between the moon and Jupiter, but from my perspective of jumping up and down, even with Air Jordans on my feet, they are equally unreachable. R' Mendele m'Riminov may be the moon, not Jupiter or Alpha Centauri, but it doesn't change the fact that practically speaking, no matter how high I jump, the world he lived in and the Torah he understood may be beyond me. A segulah is from toras hanistar; it's not like pshat in a Rambam where you can argue as to what makes better or worse sense. It would be the height of chutzpah to think I can decide whether there is anything to this segulah or not, much less to throw around terms like "darkei emori" to describe it. If I don't understand, it means my understanding falls short of the mark -- not the other way around.