(Someone suggested that the proper, though ironic, title for this piece should be
The Yerushalmi (Moed Katan 1:7) says that this passuk, in which Lavan told Yaakov to wait to finish the Sheva Brachos for Leah before marrying Rachel, teaches us that Ein Me'arvin, אין מערבין שמחה בשמחה, that we may not mix Smachos, that we cannot mix celebrations. One may not make a sheva brachos for two wives together. This has application for Yomtov as well, because celebrating a wedding on Yomtov would be me'arvin. Many hold that this is De'oraysa (e.g., Tosfos Moed Katan 8b DH Lefi and Kesuvos 47a DH Demasar, which most of you remember, and the Gemara in Chagiga 8 or something where it's also clear.)
On the other hand, the entire idea that we celebrate sheva brachos is derabanan, certainly after the first day (Magen Avraham 546:4.)
1. It's odd that we learn the de'oraysa that ein me'arvin from the sheva brachos story of Yaakov by Lavan, when the sheva brachos itself was derabanan. It's odd for two reasons.
Reason one: because if the sheva brachos is not de'oraysa, why would the Torah teach us about the inviolability of a simcha when the simcha had no halachic standing, and is, essentially, trivial. It's as if the Torah would have taught us ein me'arvin because Lavan said "Fine, you can marry my daughter, but I don't want to make the wedding until football season is over."
Reason two: There are two concepts expressed in this story: Sheva Brachos and Ein M'arvin. If you can't learn the din of sheva brachos from this story, then you shouldn't be able to learn anything at all. Or: if you say you can learn the din de'oraysa of ein me'arvin from the story, then you ought to be able to learn the din of sheva brachos too.
2. Also, the Mareh Hapanim in the Yerushalmi in Moed Katan asks, that the Yerushalmi a few pages before, in 3:5, as brought in Tosfos Moed Katan 20a, says that you can't prove halachos from pre-Matan Torah events. Here, we seem to be doing so.
3. The Rambam says that it's muttar to get married Erev Yomtov. But if we learn Ein Me'arvin from Lavan's instructions, then it ought to proscribe marrying and entire week before Yomtov. Otherwise, Yaakov could have just waited till that night to marry Rachel, not a whole week. You see that the Sheva Brachos of Leah, and the din Simcha that they entailed, precluded the Chasan from marrying Rachel, because ein me'arvin.
You might answer that the Yerushalmi holds you can't get married a whole week before Yomtov. You would be wrong. First, the mishna says "B'Moed," on yomtov, not a week before the mo'ed. Second, the Rambam in 10 Ishus brings the drasha from the story of Lavan, and of course, the Rambam holds you can get married within a week of Yomtov.
4. If Ein Me'arvin prohibits marrying on Yomtov, why does the Rambam in 10 Ishus allow marrying many women at once? True, the Rambam says that you will then have to set aside sequential weeks for each bride's sheva brachos. But what about the marriage itself? Just as one cannot marry on yomtov because the simcha of marriage distracts from the yomtov, one should not be allowed to marry two women at once, because each simcha detracts from the other?
Anyway, let's focus on the first questions. How can we learn from the Lavan's words that Ein Me'arvin, when we do not learn from his words the din of Sheva Brachos. And if we're not learning the din of Sheva Brachos, how can we learn the din of Ein Me'arvin?
The answer is the following:
The Rambam says in I Aveilus 1
So it is clear that although the concept of Shiva and Sheva Brachos are mentioned in the Torah, they do not have halachic standing; it was Moshe Rabbeinu that gave them the force of Rabbinic Law. Isn't it odd that these things were known and followed long before, and that the Torah did not give them the force of Torah law, but that immediately upon the giving of the Torah, Moshe Rabbeinu saw fit to give them the force of Rabbinic Law? And that the twin to Sheva Brachos, ein me'arvin, stated in the same context, is a halacha de'oraysa?
The explanation is that the events that preceded Matan Torah cannot have the force of law. Only the Torah is the source of law. But if the Torah describes an event, or quotes some individual, this description must have a purpose. The purpose might be to limn a righteous or a wicked man, or a miraculous event. But where that doesn't apply, then the Torah is stating a truth, a fact, a reality. The reality is that a life-changing discontinuity, an emotional upheaval, necessitates a week of adjustment and assimilation. This is not a halacha, it is a statement of fact, and it applies not only to Shiva and Sheva Brachos, but to all equally powerful experiences.
So although the seven days of celebration after a wedding, and the seven days of mourning after a funeral are realities and necessities of human existence, they are not necessarily halachos. Here's an example: The Torah teaches us middos tovos and avoidance of middos Ra'os. The Ramban says that one can technically fulfill all the mitzvos and still be a sheigitz, a naval. How can it be that we are absolutely obligated to have good middos but the Torah doesn't call them Mitzvos? Because that's the nature of the Torah. There are mitzvos, and there are facts of life and of existence that are fundamental, but remain outside the rubric of mitzvos. Sheva Brachos and Aveilus fall under this heading.
Another statement of fact is that when a person celebrates two joyous events together, each one distracts from the other. You might think that the two would reinforce each other, just as a festive meal enhances a holiday. So the Torah tells us that no; when the other simcha is for a different reason, it only distracts from the holiday, or from the other celebration.
These are two statements of metzius, fact, not halacha. Halacha cannot be derived from statements that predate Matan Torah. But the reality of ein me'arvin has halachic relevance. There are events that the Torah says require simcha, such as Yomtov. Part of that mitzva of simcha is to not do things that will interfere with the simcha. Since the Torah teaches the fact that two simchas work at cross purposes, the halachic result is that Ein Me'arvin.
On the other hand, we cannot derive any rule from the fact that powerful emotional events need seven days of assimilation. This may be a fact, but we cannot make it into a halacha. But Moshe Rabbeinu decided to help Klal Yisrael to live healthy emotional lives by formalizing this idea, but only as a Din Derabanan.
As for questions three and four, you will find a fine answer in the journal Hapardes No. 43 Vol. 7 page 23, in an article written by Rav Yisrael Meir Karno, who grew up in Kelm, as did להבחל"ח my mother, and who was one of the witnesses on my parents' kesuva in Samarkahnd. (The other eid was Reb Chaim Stein.) The basic idea is that there are two dinim, one that he will forget the primary simcha, and one that each diminishes the other. See it inside.