First, the whole premise of the thread is spilling handwavium - the British and French simply weren't going to give Hitler a free hand anywhere in Europe after he invaded the rump Czechoslovakia. So the OP should explain just why they didn't declare war on Germany after the invasion of Poland.
2nd, if Stalin doesn't expect Hitler to be bogged down in a war in Western Europe then he's unlikely to agree to a grand bargain with Germany in general or a partition of Poland in particular, meaning that the Red Army will enter the country soon after the Wehrmacht (and sooner than it did in OTL), with or without Polish consent, and it will not be a friendly operation in support of German expansionism. When the 2 armies meet the result will be either war or the establishment of a line of control (which will be different from that agreed by Molotov and Ribbentrop in OTL or its September 28 alteration). Wherever Hitler will launch the invasion of the USSR from, it will not be the Molotov-Ribbentrop line and it will certainly not be the Treaty of Riga line.
3rd, if the Germans and Soviets don't come to blows in Poland, the Soviets will attempt to obtain basing rights in Finland and the Baltic States, as in OTL. Unlike in OTL they will not have German consent in this, which could lead to a scenario in which the Germans promise to support these states in resisting the Soviet ultimatums, and if they deliver you will have your war in the late autumn of 1939. I consider this the probable scenario when the Soviets make their demands on Lithuania, which is right next to East Prussia.
4th, without the fall of France and the June 26 Soviet ultimatum expect Romania to do all it can to maintain neutrality. Without the Winter War expect the same of Finland.