POLISH BATTLEPLAN:
-FOLLOW the Weygand plan: abandon Poznan and concentrate your efforts in defending a VISTULA-WARTA line, thus shortening your frontline. The Lodz Army MUST NOT be so damn close to the border. The Pormoze Army MUST NOT be in the Polish Corridor but behind the Vistula.
-Poland must mobilize earlier and thus having 30 Infantry Divisions, 9 reserve Divisions, 3 Mountain Brigades. Thanks to more tanks, Poland COULD in theory convert 6 Cavalry Brigades into 6 Motorized Cavalry Brigades, each with one tank battalion and 2 motorized cavalry regiments (basically battalions). The 7 others brigades remain classical Cavalry Brigades. These 39 Divisions and 16 Brigades are divided between the 6 Armies of OTL.
-Prepare to blow every bridge in Western Poland. Summer was hot in 1939 and some rivers had dried but it will buy time as the German Panzers will be slightly slowed down.
-Romanian Bridgehead as OTL.
I still insist on the offensive against Eastern Prussia. It's not important that crack units be used there, nor that it takes Koenigsberg; it's important that the Germans lose ground, fear losing more, and are seen doing so.
-Convince Gamelin to be a little but just a little bolder with the Saar Offensive.
More about this below.
POSSIBLE OUTCOME
-Poland will fall.
What you should really aim for is a lucky sequence of low-probability events. Yes, low-probability, but the German success in Norway also wasn't something I'd bet upon. Thus:
- First week. The Germans largely don't advance as fast as per OTL, take losses, and where they do advance they don't smash Polish units. They also begin losing ground in Eastern Prussia and get bombed in Berlin.
- Second week. The French attack. What motivates them to be bolder isn't some Polish entreaties and pleas, it's the better than OTL Polish performance until now. The French offensive is only slightly more successful than in OTL, but the Germans do worry about it. The Poles, meanwhile, successfully delay the Germans and launch local counterattacks.
Now, for the delicate concatenation of events:
- Third week.
1. because of the above, Stalin decides to wait a bit more before getting involved, and
2. because of this, the planned larger French offensive does get launched; it makes more inroads, albeit in unimportant areas of the Saar, and
3. the German leadership begins to panic. Contrary to the Uebermensch tale, they were perfectly able to, when things did not go finely for them.
- Fourth week. Stalin, seeing the significant French commitment and the Polish stand, decides to postpone again. The French run out of steam, but the Germans are already moving troops from the central reserve to the Western Front - they have low-value, poorly-equipped 4th-Wave units to do so. They also decide to temporarily stop the offensive in Poland and to reinforce the East Prussian garrisons.
- Fifth week. The frontlines congeal everywhere. The Germans have run out of 37mm tank and anti-tank gun ammo, and their Panzer force is seriously attrited. They can still win, but they realize they'll have to revert to mainly infantry-and-artillery tactics. They are also pretty shaken.
Time for the Allies to make some serious proposal to Stalin.
Mind you, it's not likely. And even once all the above has happened, the Germans can still win, in more time. But this sequence has a chance of happening, with all the changes mentioned above.