There was nothing America or the UK could have done to keep the Red Army out of Eastern Europe - Yalta or no Yalta.
Remember, Japan was still fighting us hard in the Pacific - we only engaged a small number of her military in the Island hop war. Also Eastern Europe was not well liked by the Americans - most of Eastern European countries were members of the Axis powers - no American cared to risk their lives to save Hilter's buddies from the Soviets' wrath.
The only nations we really cared for in the East were Poland and the Czech part of Czechoslovakia (the Slovaks were Nazi allies). That is the historical fact without the modern revisionist spin.
I think you are at the wrong party!
If Roosevelt had taken Gen. Patton's advice, this story wouldn't even be an issue.
Although I don't share your fatalism about the Yalta conference, your analysis of the situation globally at the time certainly rings true, and actions taken need to be evaluated in light of those times, not the present. No one was about to risk any more American lives for the sake of the Bulgarias (or the Latvias) of the world.
Nevertheless, it is bracing to hear the American President announce to the world that the structures of a specious stability are no longer a legitimate substitute for the freedom of self-determination.