Is there any scenario in which France and England...
ITYM France and
Britain.
declare war on Germany and Poland can be saved? Could the BEF and French Army accomplish something like that? And could it be done even if Russia invades Poland from the east?
The Western Allies were in no position to do anything significant against Germany in 1939. France was only partially mobilized (and well behind Germany). The claim that Germany was defenseless in the west in September 1939 has been exploded; there were enough German divisions in the west to man the fortifications on the French border in strength.
The French army did not have the massive superiority in numbers required to smash through the German forces facing west, and that's before considering the weaknesses of French doctrine, organization, and equipment.
The French plan (such as it was) called for nothing much to be done until the entire army was mobilized and deployed on the border, sometime in late October. It was expected that most of the German army would still be tied down in Poland. Then and only then would the French attack.
But by mid-September, Poland was broken, leaving only a mopping-up. German troops were pouring into the west. The French abandoned any thoughts of attacking. By the time the French were fully mobilized, Poland was finished and the whole German arny had gone west. There was no longer any opportunity for an attack (if there ever had been).
As to the British army - Britain had only a modest (but high-quality) force to contribute to the campaign, and it wasn't deployed in France till after Poland was lost.