If the French simply overwhelm Germany's forces on the Rhine and in the Rhineland in early September 1939 then i think all historians agree the German war would be over pretty quickly. Hitler would most certainly be overthrown by the army.
What sort of peace the French and British would settle for is very difficult to know.
However these are the most favourable circumstances for an Anglo French declaration of war on the Soviets.
Even so i think it highly unlikely, there was real public hostility to the idea of war with the Soviets, there's the problem of war aims.
If Germany's Rhine front collapses quickly enough ( almost a fore gone conclusion ) then Hitler could well be gone before the Soviet's attack Finland - in which case the Soviet's are unlikely to attack Finland.
If the French move into the Rhineland quickly enough they may forestall the Soviet invasion of Poland - Stalin seeing the writing is on the wall for Hitler likely won't attack Poland.
However if German forces on the Rhine collapse and the Soviets still invade Poland then the French and British may then continue the war against the Soviet's.
Unlikely though - Stalin was very frightened of a war with Britain and France and despite his dictatorial ways was probably the most cautious of the wartime leaders.