How would you deal with the Soviet invasion that OTL ruined those plans? Are you saying the greater Polish preparation deterred them? But wouldn't that have consequences for the Molotov-Ribbentrop Pact that would itself cause huge changes in all the leadup to the war?
The main problem for the Lwow brigdehead was not really the Soviet invasion - it was just the straw that broke the camel's back. The Germans had already encirkled the Poznan and Pomorze armies in the Battle of the Bzura. The small remnants that got away retreated to already surrounded Warsaw.
What remained of the Modlin army was surrounded north of Warsaw, while Army Lodz and Army Krakow had been destroyed in the field. Army Prusy did not have time to form properly before being over-run.
What was left was Group Narew and Group Tarnow and Army Karpathia which could have reatreated to the Lwow Bridgehead. Group Narew was cut off by the Soviets. The bridgehead was not viable with less than 10% of the original Polish army - but it was the Germans that destroyed or cut off 80% of the Polish army, the Soviets cut off far less.
By the scrambled and incoherent Soviet invasion, it is pretty evident that Stalin intended to let the Germans finnish off the Poles and walz in and take his part once that was completed - the Germans destroyed the Polish army in 2-3 weeks rather than than expected 3-4 months. If the Germans are unable to destroy the bulk of the Polish army and it retreats to the Lwow brigdehead, Stalin will probably wait it out. If Germany collapses, he can claim he never intended to invade Poland, if Germany wins, he can take his part of Poland while the Germans turn on France.