Actually there was no reason that the Monarchy couldn't have been restored. There might have been some problems if Kaiser Wilhelm II or his son got the post but if one of the Grandsons got the title there probably wouldn't have been the problem. Without a doubt whoever was Kaiser would not have had the same powers that Kaiser Wilhelm II had in 1914. But at the same time whoever was Kaiser would have probably blocked Hitler from coming to power. There is always the possibility that the Royal House of Bavaria could have replaced the Royal House of Prussia. Crown Prince Ruphardt was quite popular with the soldiers in the Army and was a no nosense leader. He did not like the extreme Right or the Left.
I see some problems with a Bavarian catholic.
Prussia is far to large, and it is really difficult to split it up. And the next king of Prussia would be nearly as powerful as the new Bavarian emperor.
And the king of Prussia would control Berlin. Therefore the Bavarian Emperor wouldn't even control the capital. If the new king of Prussia is someone from the Hohenzollern dynasty, he perhaps want to reestablish his house on the imperial throne. Therefore the weak Emperor and the Prussian king could blockade each other.
At the same time, at least some constitutional changes for a constitutional monarchy are nearly inevitable.
If the conflict between Emperor and the King of Prussia escalates, a strong SPD could try with the other left wing or republican parties to abolish the Monarchy, other far more radical left wing parties could try to start a civil war. And in this situation, perhaps some talks between military leaders and the leading parties could decide the future of the Empire.
To prevent this civil war, it is either necessary to either split up Prussia (unlikely), reinstall the House of Hohenzollern as emperors, or abolish the monarchy (unlikely for the conservative politicians).
It is difficult to say, that an Emperor could have blocked TTL-Hitler or some other form of fascism.
An Emperor could try to regain power by cooperation with different right wing circles and militias. Something like in Italy could happen. If a fascist party would win the elections like OTL the NSDAP, the Emperor could become TTLs Hindenburg. Either he tries to weaken the constitution and appoint his own government, against the parliament, or he allowed some fascist government. (Like in Italy)
If TTL-Emperor tries to stop some form of Fascism (if this Fascism is as popular as Fascism was in the end of Weimar Germany), he needs the support of different groups, he needs a large alliance of the democratic parties, (OTLs SPD, DDP, Zentrum, and DVP) and the full support of the military.
There is a reduced chance of Fascism in Germany, because to maintain the Monarchy after WW1 in Germany, a weaker treaty of Versailles is almost necessary.
Also Butterflies would reduce the probability of events like OTL.
Therefore a weak monarchy isn't the best protection against Fascism.