First, a brief history about prior attempts to decipher who is the real author of Mekore Minhagim is in order. As I noted in my original post, the first edition of the sefer to come out was published in Berlin in 1846 with the author listed as R. Avrohom Lewysohn (1805-1861). That edition contained 100 questions and explanation about various customs. Then, in 1851, R. Yosef Finkelstein published under the same title a work with the very same information, but that contained only 41 of the 100 questions and explanations from the work published in 1846. Almost immediately, it was claimed that Finkelstein had plagiarized his work from Lewysohn. And if one had to guess – absent any additional information –it would appear that this is the case simply because Lewysohn's work came out first; that is, unless Lewysohn could have read Finkelstein's mind, the latter must be the plagiarizer.
But, this is not a simple case. Instead, the appearance of the plagiarism claims in a German periodical did not settle the issue. Thus, R. Lewysohn's brother, Yehudah Leib Lewysohn, a Rabbi in Stockholm, after seeing Finkelstein's name mentioned in a different capacity in the journal ha-Maggid, again pointed out that Finkelstein had plagiarized Mekore Minhagim from Lewysohn. R. Y.L. Lewysohn gave a run down of the controversy and included the fact that, eventually, the dispute was taken to court, which ultimately concluded that Finkelstein had plagiarized from Lewysohn. But, it seems that Finkelstein had someone swear on his behalf that he was indeed the author.
After R. Y.L. Lewysohn published that account, including the court case coverage, Finkelstein himself answered the charge in a later issue of ha-Maggid. Finkelstein claimed that he was indeed the author and Lewysohn had stolen from him. But, how to account for the fact his sefer came out later? Finkelstein claimed that as he was traveling through Germany, he stayed with Lewysohn and eventually showed him his (Finkelstein's) manuscript of Mekore Minhagim. Lewysohn was extremely taken by this book. According to Finkelstein, Lewysohn must have copied his version and published it before Finkelstein was able to.
R. Y.L. Lewysohn responded – with a point by point rebuttal – that Finkelstein’s account was all untrue and challenged Finkelstein to go in front of a court again - but this never happened.
That is more or less a summary of the written record with respect to the controversy. So it seems there remains the possibility that Lewysohn did copy Finkelstein’s manuscript when they met in Berlin. And, in fact, many have come to Finkelstein's defense. For instance, R. Tzvi Efraim Babad in Der Yid has an article where he uses the ha-Maggid article to show that Finkelstein was indeed the author. In particular, it seems that R. Babad didn't like Lewysohn, as he was a German Rabbi and university educated, while Finkelstein was from a distinguished rabbinic Hungarian family. There is also an article in the latest Or Yisrael about this incident of plagiarism.
I think, however, that I can prove who the real author is. I can do so by using Finkelstein's own defense from ha-Maggid to demonstrate that he, in fact, is the plagiarizer. As is many times the case, he created the noose by which to hang himself.
Finkelstein, in his defense, states as follows:
When I was in Prague I wrote the work "Rivid ha-Zahav" which discusses the laws of ritual slaughter and checking for imperfection of the lungs. Many great Rabbis praised this work amongst them the famous Gaon R. [Shlomo] Yehuda Leib Rapoport and, because so many people liked it, the book sold out and I had to publish it again. After this I published another book "Tzafnas Panach" on blemishes in the lungs [of an animal].He then continues and discusses the “Mekore Minhagim” and how Lewysohn got it:
When I traveled to Germany to sell my book I stayed with [R. Lewysohn] . . . when he saw my work the ‘Mekore Minhagim,’ which I wrote in 1839, he asked to look at it.From there Finkelstein posits that Lewysohn eventually copied it and printed it as his own.
So, now, in order to see who is actually right, we need to see if R. Finkelstein's story works. The way to do this is to check the books that Finkelstein actually was selling. First, it is important to know that Finkelstein published three books aside from Mekore Minhagim. As mentioned above, he wrote Rivid ha-Zahav and Tzofnas Panach. In addition he published a book his father- in–law, R. Meir Avraham Csaba, wrote - Pri Tzadik. Pri Tzadik was published in 1839, Finkelstein’s first published work. Now, according to Finkelstein, in his response in ha-Maggid, he published Tzofnas Panach after he published Rivid ha-Zahav for the second time. So, that would make Tzofnas Panach the last book published. Also, according to Finkelstein there were two editions of Rivid ha-Zahav (these are the only editions of Rivid ha-Zahav) but when was Rivid ha-Zahav published? According to the title pages, one was published in Prague (1846) and the other in Ofen (1845). But, according to Finkelstein's own testimony, these dates must be wrong -- or at least one. The reason being, if you recall, is that Finkelstein said Rivid ha-Zahav was written in Prague and was praised by R. Rapoport -which you can see as there is an approbation from R. Rapoport. In particular, the first edition of Rivid ha-Zahav has this approbation according to Finkelstein's own words. But, the only edition which has this approbation is the one with 1846 on the title page and the approbation itself is even dated the 6th of Av 5606 (1846). That means that, although the other edition of Rivid ha-Zahav states was published in 1845, in fact, it was published after the 6th of Av 5606. Which also means that Tzofnas Panach was also published sometime after the second edition of Rivid ha-Zahav was published.
Now, for Finkelstein's story to be true, he states that he was selling "his books" -"ספרי" that means his personal books. That means we can rule out Pri Tzadik as that was his father-in-law's book and Finkelstein wouldn't have called it "his." So when did he travel to Germany to sell his books and to which books did he refer? Well, let's take the earliest of his books - which according to what we have figured out - is the first edition of his Rivid ha-Zahav. That edition of the Rivid ha-Zahav had to have been published sometime after the 6th of Av, the time of the approbation. That doesn't leave that much time in the year 5606, being that Av is the second to last month in the Jewish calendar. But, let's say he had Rivid ha-Zahav published really fast and during the month of Av he was able to publish it and was already in Germany meeting up with Lewysohn. Well, and here comes the funny part, Lewysohn's introduction to Mekore Minhagim (which is copied in Finkelstein's as well) is dated 16th of Kislev 5606, which would be around December 1845. This, of course, means that if our calculations are correct and we take all of Finkelstein's story as true, Lewysohn wrote the introduction at least ten months before Finkelstein ever came to town to sell his then, unpublished, Rivid ha-Zahav. Which means Finkelstein is a liar.
Thus, it would appear that we can now conclude who is the plagiarizer – Finkelstein. And, the fact is that Lewysohn is the real author of Mekore Minhagim.
 There is another reason the Tzafnas Panach must be the final book published although again according to the title page there is an earlier date. According to the title page it was printed in 1845, but now that we know the 1845 edition of Rivid ha-Zahav was in fact published after 1846 the Tzafnas Panach must also be published after that. This is so, because in the Rivid ha-Zahav with the title page which claims 1845 it also says the approbations for this will be published in my future work Tzafnas Panach (which in fact Tzafnas Panach includes). Thus, Tzafnas Panach must be after this second edition and thus must be after 1846 even though it claims an earlier date.
Sources: ha-Maggid No. 24 June 17, 1863 p. 192; No. 27, July 8, 1863, pp. 211-12; No. 36, September 9, 1863 pp. 283-84; No. 40, October 14, 1863, p. 316 (which are all available online here); R. Tzvi Ephraim Babad, "Printers, Copiers, Shasin, and Censor," Der Yid 25 (Friday, September 22, 2000), section 2.