So:
- Without the Soviet invasion, could the Romanian Bridgehead have held until the French mobilised?
- What else could the Wallies have done in the meantime?
- Without the MR pact, would the Nazis still have invaded, and could the Soviets have afforded not to? I take it there's no way they would have come in openly in support of Poland - memories of the aftermath of WW1 were too recent.
- Given a 'neutral' Russia, how does the war develop?
1. I think Poles could have hold for some time. But not forever.
IOTL about 60 000 - 70 000 Polish soldiers crossed the border to Romania and Hungary (not to mention many more civilian refugees); ITTL that number would be significantly bigger, since many soldiers trying to break south were stopped by the Soviets; also some units might decide to march to Romanian border, like Independent Operation Group "Polesie" led by general Kleeberg with 18 000 men (Originally Kleeberg was ordered exactly to go to the bridgehead, but after Soviet invasion and some fight with the Red Army he decided it was impossible to get there and turned towards Warsaw). There were also units defending Lwów. IMO Poles could have easily gathered 150 000 soldiers there, possibly more.
Potential problems
Supplies. I doubt the Allies would have been able to organize a supply line to the Poles form Romanian ports - remember, that Romania was formally neutral; add to that a time Allied ships would need to reach Romania. However, many transports with weapons, ammunition and other equipment from garrisons in eastern Poland were directed to the bridgehead. IOTL many of them were captured by the Soviets, ITTL at least some of them might reach their destination. I believe that those supplies might allow the Poles to hold at least for 3-4 weeks, possibly longer. IOTL Germans also had logistical problems - not as much about lack of supplies (although I heard they have some problems with fuel) but with getting them to the frontline. ITTL their supply lines would be even longer and lead to the less devloped part of Poland with fewer roads and railways.
The main problem would be organization of the Polish defense. Polish command would need to very quickly reorganized available units into a force capable of stopping the Germans; I think there were enough officers to do that (hey, Polish General Staff and Command in Chief were there), but it might be a serious problem. It is possible that defence of Lwow (vital communication hub, among other things) might have given Poles some time.
2. Start bombing Germany for real, insted of leaflets; send the supply ships anyway, at least some of them might arrive in time. And continue that Saar offensive
3. That is debatable; even Hitler was worried about British and French actions, and an alliance with Stalin gave him a strong confidence boost. Not to mention Germany would need to prepare for a possibility of a Soviet action against them. Not very propable, but possible.
4. Poorly for Germany, I think, even if they eventually crush the bridgehead and Soviets decide to remain neutral. First, the campaign in Poland would be at least a few weeks longer and somewhat bloodier. Second, they would need a much larger force in Poland for occupation purposes and to keep an eye on the Soviets. And most important: IOTL the Soviets provided Hitler with thousands of tons of food, strategic raw materials and oil in exchange for German technology. I doubt it would happen ITTL with both Soviets and Germans at least very suspicious about each other. Hitler would not give Stalin his newest toys; Stalin would not give his resources for free. And Hitler had no cash to pay for it. Add to that allied blokade and German economical problems would be enormous.