Hitler was surprisingly pragmatic. Or rather unsurprisingly pragmatic, you just can't gain control of Europe from the Atlantic to Moscow if you aren't pragmatic. In OTL he "offered" to recognize Poland's borders in return for the Poles acting as cannon fodder in his next war. To get Poland to agree to this would need:
1. No guarantees to Poland in 1939, or Poland not underestimating the chance that Germany and the USSR would attack. The government of Poland did not expect a two-front war.
2. The extremely anti-German public opinion tolerating this. Authoritarian governments are slightly better at opposing public opinion then democracies, but even they have limits.
Would Germany still attack in the west if it's allied with Poland?
Definitely. Possibly even in 1939. Germany wasn't fighting for Danzig, it was fighting for world domination. If this invasion of France is stopped we would get an interesting situation. A Polish stab in the back might significantly shorten the war, but Stalin might also use the opportunity to attack while Germany, Britain and France are all distracted.
And how long would it take for Poland to be subjugated to Germany after the said victory in the east?
In terms of foreign policy Poland would be expected to do what Germany asked from the very beginning. The late 1938/early 1939 German demands put it in slightly nicer words, but their meaning was clear.
going that route myself. Poland was essentially a quasi fascist state in the 1930s (as was Greece and Portugal).
There was no supreme leader. There were elections, something intolerable in fascism. Opposition parties were prevented from coming to power, but nobody questioned their existence. (The only exception was the Communist Party, but they were being controlled from Moscow so that was necessary.) If this is quasi-fascism then it seems that quasi-fascism can mean almost anything.
The Poles did little to protect the Jews and in fact for the most part were willing to let them be 'resettled"
The government would have wanted a Jewish state in the middle east, but deportations and mass murder are completely different things. Under German occupation the penalty for any sort of assistance to a Jew was death. Under such conditions one might as well criticize the Jews for not protecting the Poles under these conditions.