I think you are playing Version 2, right? My assumption is based on your VP question, as that VP condition changed a bit in Version 3. Regardless, I think my answers are all relatively the same, even though I don’t have the Version 2 rules in front of me.
So, in Version 2, the winner of the Spanish Civil war gets that VP. Doesn’t matter if all of Spain is overrun by another power later on. The winner of the war gets a point the moment the war is won. Version 3 changed the rule to lump it in to the Worldwide Communism rule. So the rule is almost the same for the Comintern in that the IPP territory in Spain would count for them if they win the war, albeit they’d have to hold on to it by games end.
So, you might be misunderstanding the rule a bit too. First, the game starts with the Spanish Civil War happening already. So your thought of the Spanish Republicans aligning to the Comintern is incorrect in that regard. The Spanish Civil War is fought internally until one side wins or certain things happen. In Version 2, if the Spanish Civil War is still happening once Germany and the USSR go to war, both factions align to their respective powers, even though the war isn’t resolved. If the war is resolved before Germany and the USSR go to war, then the winning Spanish faction is just a controlled minor power of the victor until it’s attacked or other conditions happen. In Version 3, it looks like this rule changed a bit. the Spanish factions stay as controlled neutral minors unless attacked later on.
To your example of the British attacking, I’m 99.9% sure you cannot do that. I don’t have the Version 2 rules handy, so can’t say with certainty. But I believe no one can interfere in Version 2 outside of Germany, Italy, and the USSR sending Lend Lease. The British could attack a Nationalist Spain that won the Civil War, but couldn’t help while the war is ongoing. That’s if I’m remembering the Version 2 rules properly.
In Version 3, it’s not quite as clear, but I think is implied. The rulebook at 13.1 Spanish Civil War under Participation says: "Spanish factions are controlled Minor Powers that cannot declare war on other nations. Germany and USSR are not at war with Spanish forces and cannot attack Spanish units or land zones (or vice versa). So, this specifies directly that Germany and the USSR cannot directly fight in the Spanish Civil War. I think it’s implied that no other nations may attack either while the war is ongoing, but I certainly could be mistaken there.
Besides all that, I’d think twice on your strategy of “England to manipulate the war in favour of the Comintern”. I only say that because, as the Allied player, the UK would potentially be taking a VP away from ally France. France has a VP of Eliminate Communism, which reads: “Score 1 victory objective if the Comintern does not Possess any land zone adjacent to French Home Country. A neutral Republican Spain counts as Comintern land zones.” So, if you help the Republican’s win, France would have Communists knocking on the back door!