You say it a lot, but to me it seems much more British wishful thinking than anything else. What's in taking part in BoB for the Soviets ? Well, for once they keep their ally happy by showing serious and productive committment at hitting the common enemy,
Keeping rabid bloodthirsty Slavophobes scheming to betray you happy: number one priority of any sound Russian policy.
Stalin said, in 1940, "These Nazis aren't to be trusted and we work with them in order to buy time". What's the more effective way to keep the Germans off your back: honey up to them to show them they were wroung about Jewish-Bolshevist Untermensch all along (and in the process increase your own military vulnerability to them), or leave in existence a credible threat on their other flank? The answer would be obvious even
without factoring in Stalin's religious, mistaken belief that Germany would never fight on two fronts.
and not letting their otherwise estranged ally do all the heavy lifting,
That was
precisely the point.
in a way that does not tax Soviet military resources very much.
The less it taxes them (and every little bit it taxes them is a little bit less to defend the Rodina), the less it taxes us. We won the BoB pretty convincingly.
So they have to deploy some of their air assets to France ?
Remember that
any and all assets deployed to France will vanish the second Germany turns against the Russians. Which they will, and Stalin said so in December 1940.
Well, it's not like Soviet war effort is going to have much else to do, after the Middle East is conquered, besides replacing those planes and pilots.
Prepare for the confrontation against Germany, maybe? And why should they have to do anything? It's not like they wanted a war with Britain and France.
Of course, the Soviets might also keep pushing towards India,
They might also invade the moon and conquer the bottom of the sea. Sorry, but Russia invading India is a feverish fantasy of the original Jingos. It doesn't work, logistically speaking.
but they would do it for their own interests, since this is an area they coveted as well.
I have yet to see evidence of this. The Azis negotiations you like to cite consisted, it seems to me, of Ribbentrop going "Look! India! Shiny!" in an attempt to distract the Soviets from Germany's diplomatic preparations for Barbarossa (Germans in Finland but no Russian in Bulgaria), and Molotov not buying it.
As for Turkey, I do not expect it is going to last very much, after the Germans attack it from the west,
Bulgaria may not be mustard-keen on this idea.
the Soviets from the north and east,
The "east" was a narrow, mountainous frontier where Turkey had the interior LoC. The "north" was a big ole' sea. The Red Black Sea Fleet wasn't up to much, and if the Soviet Army (Winter War Vintage!) was capable of amphibious operations, which I doubt (or would they not have simply sailed into Helsinki in early December '39?), it would only capture a thin coastal strip before the mountains.
How?
Afghanistan is also the door to India.
Yep! All you need need to do is to send multiple divisions across a part of your own country with comapratively minimal military infrastructure, into a pre-modern kingdom containing sweet fanny adams save mountains and angry armed people who hate you, over some narrow mountain passes, and up against fresh enemy divisions.
Easy job, really. It's just as well you don't have to defend your motherland from a foe determined to invade and kill every single one of your people.
Germany can easily hit Turkey itself from Bulgaria.
If Bulgaria says so. Yugoslavia, Romania, and Greece still exist at our PoD.
It's not like they are not going to have spare troops, after France and the British Middle East are done with. Germany from Bulgaria, Russia from Caucasus and Iran-Iraq, both from Syria.
Where did they get Syria?
How long is Turkey going to last ?
Not forever, certainly, but hey, Finland astonished the world, and it doesn't even have mountains. For a sufficient length of time for it to be questionable whether the Soviets would bother.