It would have been hard to find any significant group of Poles actually willing to collaborate with Germans. Sure, Polish nationalists detested Jews and communists, but they weren' exactly fond of Germans either. Anyway, everybody would know that Western Poland is simply a German puppet and its population only waits for a good moment to put a knife into German back. Germans would never be able to trust them.
In eastern Poland I'm not sure if Poles would be to biggest national group - I think Ukrainians might be more numerous. Therefore making it a "Polish Soviet Socialist Republic" would be absurd - especially since a significant part of Poles in those territories were "enemies of the working class" - members of the middle class, landowners, businessmen, soldiers and administration. Such people couldn't count on lonmg life in USSR. Had Poland been divided according to first version of the Soviet-German pact (with the boprder on the line of Vistula, more or less), that would be different.
Western/Eastern Poland as a buffor state between USSR and Germany wouldn't work either, - both Polands would be puppets, with significant numbers of German/Soviet troops occupying them. Situation would be more or less the same as with actual annexation. Not to mention the fact that the Nazis weanted their Lebensraum, and Poland was not enough for them.