...I doubt that the Head of the House can adapt the line of succession if he wants...
I think you'll find that German law won't get involved in such matters. So long as estate inheritances are distributed according to the law, the internal matter of who shall become 'Head of the House' and receive the family patrimony is guaranteed by a valid will or testament and to some extent family tradition, and that's the end of it. This principle was established in two similar cases - Leiningen and Hohenzollern.
Emich, the 7th Prince of Leiningen removed his eldest son, Prince Karl Emich, as Hereditary Prince due to his 'unequal' marriage and more or less wrote him out of his will (and/or the Leiningen Family Trusts). Karl Emich challenged this action in court and lost; the court upheld the right of the Fürst to determine how the family patrimony could be distributed or passed on and in due course the second son, Andreas, succeeded to the Headship of the House as the 8th Prince of Leiningen and inherited the bulk of the (very considerable) family assets.
I've got more detailed information on both cases but I haven't fully indexed the publications they appear in and rather than wade through my folders I'll refer instead to these [reliable] web sources:
https://groups.google.com/forum/?fromgroups#!topic/alt.talk.royalty/xtJ_lG5Ricw
quote:
"The court was Germany's Federal constitutional court (Bundesverfassungsgericht), and the court's own press release is at:
http://www.bundesverfassungsgericht.de/texte/deutsch/pressemitteilungen/bvg24-00.html
The exact nature of the decision was to refuse to hear the appeal put forth by Prince Karl Emich against the decision of the Bavarian High Court in 1996. His appeal was based on the freedom to marry protected by the German constitution.
The constitutional court motivated its refusal by agreeing with the Bavarian High Court, and siding with the freedom to dispose of one's possessions by testament, also protected by the constitution."
The Leiningen decision came soon after the Hohenzollern decision.
The Hohenzollern case concerned a similar principle, namely that Prince Louis Ferdinand was entitled to pass the bulk of the Hohenzollern patrimony and succession to Prince George Friedrich, the son of Louis Ferdinand's 3rd son. Prince Louis Ferdinand's two eldest sons, Princes Friedrich Wilhelm and Michael, who had previously made 'unequal' marriages, were thereby passed over. They challenged Georg Friedrich in court, and lost.
Full details link:
The Hohenzollern succession and the Federal Court of Justice ruling of 1998
Putting the financial battle to one side, the Court, at least in these two cases, would not involve itself in a determination of who should bear the title or honour or style of 'Head of the House' but left it to the will or testament or previous valid arrangements of the former Head or Heads.
I'm not an expert in German law by any means but these two cases were reported in some detail in specialist royal publications at the time and while my summary may not be 100% correct I believe the gist of it is. Thus the Margrave can "pass on' the House Headship to Alexander, his legally adopted son, and whether it be right or wrong, moral or immoral, ethical or unethical etc etc, has no particular relevance. The worst that can happen is that some folks will recognise Alexander's newfound status, some won't, and the vast majority won't be aware or care.
is right: it doesn't matter one bit if it is just or not or what one personally think. It only goes to show again that there is s clear difference between nobility of birth and nobility of character. 
