The only way to get a kingdom is a CP victory. Pilsudski wanted a republic and as the powerbroker among the Polish he had the most say in the post-war government. Also the Polish Legion had a very different perspective than the Poles from Galicia or the British. They had more in common with French and US Republicanism.
If we had a CP Kingdom of Poland there could be a Habsburg monarchy depending on the specific scenario. That would be very interesting, especially as A-H was becoming increasingly hostile to German domination and would find Poland very much in the same situation. Though Poland's monarch would likely be like Finland's monarch, the links between Austria and Poland would likely strengthen against Germany as Germany pushed a Mittel Europa style system on both A-H and Poland (Hungary would relatively like that system because she could sell grain at inflated prices to a captive German market, further pissing off everyone else in A-H).