...what is to stop Soviets from rolling over? Not that it was or was not Stalin's intention, but can Hitler ever be sure it won't happen?
The intrinsic inferiority and incompetence of Bolshevik untermenschen, which Hitler recognized from the first, and which was apparently confirmed in the Winter War.
Hitler was utterly confident about crushing the USSR in a swift campaign. "Kick in the door, and the whole rotten structure will collapse."
Given that attitude, it's hardly implausible for Hitler to believe that Germany is no real danger from the USSR at that time.
As for a PoD: suppose that Sanjurjo, not Franco, was Caudillo of Spain. (The PoD is in 1936, and of course there would be butterflies, but bear with me.)
Sanjurjo was to be the leader of the rebellion, but died in a plane crash due to choosing to fly with a "daring aviator" in his small plane rather than the large twin-engine plane that was available -
and taking a trunk full of dress uniforms with him.
I don't know much else about his personality, but that incident suggests that had he lived and been in charge in 1940, he would be very likely to rally to Hitler's side and join the Axis.
This commits Germany to additional campaigning in the west (siege of Gibraltar), air/sea combat around the Canaries. It might lead to Axis control of Morocco, and maybe more. Perhaps Petain and Laval take Vichy France into the Axis to avoid further Axis domination of France - i.e. better a willing and autonomous Axis state than continued jerking around by Hitler.
With French cooperation, the Axis can deploy a lot more force to North Africa.
With all this going on, Hitler is more likely to think he can force Britain to make peace, and doesn't need to take out the USSR first. The General Staff and even Goering would rather not fight a two front war.
What does Stalin do? His goal, as he told the Politburo in 1939, was to have Germany fighting the Western Allies as long as possible. But the next year showed some ugly surprises. The Winter War showed the Red Army was seriously defective, and the French campaign showed the Germans were far more dangerous than expected. The Western Allies have been stomped, and the USSR potentially faces the entire Wehrmacht.
It's like the exchange between Gandalf and Gimli in
The Lord of the Rings, where Gimli wishes that Saruman and Sauron could fight each other. Gandalf answers "The winner would emerge stronger than either, and free from doubt." Basically Stalin has made Gimli's call, and gotten Gandalf's unwelcome result.
Stalin therefore will put off any confrontation with Germany for at least a year or two. That will give him time to consolidate and fortify his gains, and refit his army. The question then is does he dig in and wait for the Axis attack, or attempt a pre-emptive attack, and if so when?
Anothe question is how long Britain can hold out in these circumstances. Side issues include whether any additional French colonies join Free France. If West Africa (Dakar) follows Vichy orders into Axis service, Britain is seriously screwed and may break. If it flips, Britain has a decent position.
In the latter case, Stalin may decide to strike in 1942. Otherwise, I think he just digs in and waits.