During the 30's the Soviet leadership was panicked about a possible war with Germany and they proposed a collective security agreement with Britain and France, which was refused. Since the European powers saw communism as a primary domestic and international threat during the interwar period (Lloyd George and some other political figures saw Hitler as a bulwark against Russia), wouldn't they have let Germany go to war with Russia without interference?
Axis leaders would have known about the Allies' willingness to look the other way when they took military action against a common pest since they didn't object to Germany/Italy's violation of the non-intervention treaty in Spain.
Secondly, if France and Britain had signed a collective security agreement with the Soviet Union, would that possibly have lessened the destructiveness of the whole conflict, since it would have drawn the Soviets into the war a lot earlier?
In order to answer the first part of your question, I'll start by asking you a question - how would the Germans have reached the USSR? If you look at a map of Europe in 1936, you'll see that Germany does not share a land border with the Soviet Union. In order to invade, they would need to acquire one. They don't even have a land route to East Prussia at that point, so Poland is really the only sensible choice as a target to acquire an avenue through which to invade Western Russia. If you take a look at Germany's economic and military situation over the interwar period (see: The Wages of Destruction by Adam Tooze) you'll see that this was all heavily fueled by their annexations of Austria and Czechoslovakia, the latter of which also gave them a substantially larger land border with Poland in addition to it's large armament industry.
It's always hard to give firm answers on counterfactual, but it's hard to imagine Britain and France just overlooking a full scale invasion of Poland followed by a war of aggression against the USSR. I think any general history of WW2 would be fine to point you to here, even a somewhat dated one like The Rise and Fall of the Third Reich or if you didn't want to commit like that, a TV series like The World at War. The diplomatic situation between France, the UK and Germany was extremely strained and all parties were very interested in containing the Germans, even if they were hesitant to go to war. Negotiating "a free hand in the East" was a much desired policy goal of the Nazis, they would have greatly preferred this, but the French and British were not going to give it to them, especially not when it involved trampling over multiple neutral countries with whom they enjoyed good relations to get there
"why didn't x do y" questions are really difficult to answer, at least to r/askhistorians standards, because there's usually no documentation to support anything.
One thing to consider re your first question though is Poland - I'm not sure how Germany could go to war with the USSR in the 1930s without involving Poland, and that is after all what ended up drawing in Britain and France IRL.