It's interesting that the 1831 Constitution of the Kingdom of Saxony and the 1837 House Laws [see http://www.heraldica.org/topics/royalty/HGSachsen-K.htm
] actually define the "house" somewhat differently! The Constitution, as you say, settles the succession in the entire "Saxon princely house" via Semi-Salic law, which would imply that all of the Ernestine branches would need to be exhausted as well before the throne could pass through or to a female. But, the House Laws explicitly confine the definition of the Royal House of Saxony to the Albertine Line - an important distinction that the Constitution does not make! This could of course be interpreted to mean that the House Laws were only intended to govern the immediate Royal Family, but that the other Wettin lines nonetheless held collateral rights of succession to the throne in the event of the Albertine line's extinction. This interpretation is perhaps what Prince Michael had in mind when he and the heads of the other Ernestine branches objected to the late Margrave Maria Emanuel's dynastic innovations.
The problem with allowing the Margrave himself the "final word" on such matters is that his brother Albrecht [who did indisputably succeed him as Margrave and Head of the House for a little over 2 months] had quite a different interpretation of the matter, and made it quite well known both before and after his brother's death. In fact, both brothers were somewhat limited in their prerogatives here: the 1831 Constitution of the Kingdom of Saxony (Articles 6 & 7) mandated "equal marriage" as a requirement for succession. However, the actual mechanics of the dynastic marriage requirements were governed by the House Law of 1837 (amended in 1879, 1888, and 1900), and as with all house laws promulgated by royal decree they could be interpreted and revised (though not flagrantly ignored) at the King's discretion. House Laws would generally require the consent of the surviving dynasts to modify however, and this again is where it becomes important to consider the attitudes not only of the surviving agnates of the Albertine line in 1997 and 2002, but also of the heads of the Ernestine lines in 2012 and 2015.
The main point of contention here is whether the late Margrave of Meissen actually modified the standards for "equal marriage" within his house (thereby ensuring compliance with Articles 6 & 7 of the Constitution), or whether he simply attempted to designate his nephew Alexander to succeed him directly via adoption (which would have been explicitly forbidden by Article 13 of the House Laws of 1837). If the late Margrave Maria Emanuel did in fact modify the equality standards for his house, then this would necessarily apply to all marriages within the dynasty - not selectively (and retroactively) only to the marriage of Alexander's parents!
Alexander's claim holds up only if it can be proven that the late Margrave redefined the House Laws governing equality of marriage, with the consent of the family at large, to encompass the marriage of his sister to Roberto de Afif. If we argue that the equality requirement was scrapped entirely, then Rudiger is the heir. If in fact the equality requirement was never properly redefined or scrapped, then the heir is either the First von Hohenzollern (if we hold that the 1837 House Laws' definition of the Royal House as the "Albertine Line" only must implicitly apply to the constitutional line of succession), or Prince Michael of Saxe-Weimar (if we take the 1831 Constitution's definition of the "Saxon princely house" at face value, and assume it must apply to all existing Wettin branches).
I'm reserving judgment on the question unless and until I see further documentation either proving or disproving the possibility that the house requirements on equality were actually redefined. Until that happens, I'm inclined to agree with the expert opinion published by the Deutscher Adelsrechtsausschuss, which concluded that neither Alexander nor Rudiger has a clear legal right to style himself "Margrave of Meissen" or "Prince of Saxony," and that the Royal House of Saxony is now extinct: https://www.adel-in-deutschland.de/GutachtenHausWettinZwischengutachten.pdf
What seems likely at this point is that the Ernestine Wettins will eventually dwindle to one single surviving branch: the Saxe-Coburgs are the only Ernestine branch that show some promise of lasting into the next generation or so. The territorial divisions within the Ernestine Wettins were never understood to be permanent, and when one line became extinct the family as a whole would have to agree on a new territorial division between the surviving branches - but always whilst maintaining succession (to both lands and titles) within the male line only. The last such occurrence was in 1826, when the extinction of the Saxe-Gotha und Altenburg line necessitated a fresh division of territories by agreement between the surviving lines, under the arbitration of the King of Saxony as head of the Albertine branch.
We may then see the royal, grand ducal, and ducal Saxon/Thuringian heritages merged!
This is what makes Prince Michael's current position rather delicate: he and the heads of the other surviving Ernestine lines objected to the late Margrave of Meissen's dynastic innovations because they violated prior usage, and because they failed to obtain the consent of the House of Wettin at large. But now, facing the imminent extinction of his own line, he is apparently trying to designate his own daughter as his dynastic successor, replicating the same solution that he declined to accept for the Albertine branch. I would be surprised if this "solution" garners much sympathy or acceptance from the princes of the Saxe-Meiningen or Saxe-Coburg lines.
My understanding is that his daughter, Princess Leonie, is heiress only to whatever properties and fortune the family has and to any cultural positions the family exercises... minus becoming titular Grand Duchess of Saxony.
Regarding the Albertine Wettins: can we really even say that the Saxe-Gessaphes have "inherited" anything? The legal issue hinges, I gather, on whether the late Margrave of Meissen actually changed the house laws regarding dynastic equality, or whether he simply declared his nephew to be his dynastic heir by fiat. Everything I've read on the issue up to this point suggests that he followed the latter course.
Those who enjoy proximity to these descendants of the Saxon royal house seem to think the Margrave had the final work on the matter, ignoring that the last monarchical constitution settled the succession on the entire "Saxon princely house" (House of Wettin) at large.