Alternative Habsburg inheritance 1556

[SorelusImperion]

I have been working on an alt-history scenario and have reached a point where I'd like to get some outside input.

The premise is that the Reichsreform of the late 15th early 16th century was an overwhelming success and the Habsburg dynasty managed to strengthen Imperial power. Assuming that Charles inherited Spain, Burgundy and the Imperial crown we see a somewhat overpowering Habsburg Empire in Europe which is far more stable than in reality as the power of the German nobility has been reduced and while the Reformation still takes place it is far more limited and less of a threat. So we get to the year 1556 Charles still abdicates due to his bad health but instead of splitting the Habsburg Empire between his son and his brother he makes Phillipp his sole heir.

Obviously this wouldn't went down well with the German princes wo would fear of beeing completely removed from power by Philipp nor would Ferdinand be happy. So I imagine that there is a considerable risk of war between Philipp on the one hand and Ferdinand on the other but would Ferdinand actually risk open war ?

How would France, Hungary and the Ottomans react to this situation ?
Hungary beeing inherited by Ferdinand as in OTL has managed revert most of the Ottoman conquests (with exeption of Siebenbürgen/Transylvania which is still part of the Ottoman sphere). Would it support any side ?
How about France ? Obviously it has failed so far to contain Habsburg power. Attempts to support the opposition in Germany were not successfull due to the stronger Imperial control and the alliance with the Ottomans wasn't sufficient either to break the Habsburg hegemony.
The Ottomans might try to use the situation to conquer Hungary or they might simply be sufficient with replacing the Habsburgs in Hungary.

Can you think of a way how Philipp of Spain manages to win the Imperial crown without the Spanish Habsburgs becoming too powerfull to be kept in check by their rivals ?

One possible scenario would be a compromise peace between Philipp and Ferdinand within which the Imperial crown stays with the Spanish line while the Ferdinand and his descendants are placed as hereditary regents of the Empire. Is such a compromise plausible ?

Then there is Italy. As in OTL the Habsburgs manage to dominate Italy the difference beeing that a more centralized system is established and that the legitimacy of Habsburg rule in Italy is derrived from the Imperial crown. That is "Reichsitalien" has a stronger link to the Empire than in OTL with the Archbishop of Cologne being placed in a position of authority over the Imperial domains in Italy (through his position as Imperial chancellor for Italy).

As far as I can see it a cpnflict over the Habsburg inheritance has some potential to turn into a great war greatly destabilizing or possibly even ending Habsburg hegemony.