You are right, we should be blaming Americans and British for selling us in Yalta.

Eastern Europe wasn't something that the Americans and British could give away, it was the hard work of the Soviet Army (and many bad choices by Hitler) that ensured that Hungary would be on the Soviet sphere of control. Churchill did agree to the Percentages Agreement (there is no evidence that FDR ever learned about it) at the Moscow Conference in October 1944, but even that was already too late, it was merely an acceptance of something that Stalin already had.

Stalin seems, per _Yalta_ by Plokhy, to have tried to keep Poland and France balanced against each other. In June 1944 de Gaulle set up the GPRF for France, and a few weeks later the PKWN ("Lublin Government") was set up for Poland. Both started out as broad-front governments that combined Communist and Capitalist forces to fight the Nazis, and then over the course until 1945-6, when the Fourth Republic and the TRJN were created after the war ended, more normalized governments were created again combining both elements, and then lasting a few more years until the weaker side in both governments- in both cases the side not allied with the armies that liberated them- were finally fully purged from the Government. That was 1947 for Poland and 1949 for France.

Basically, I think that the absolute best case the Western Allies could have gotten, if this was their most important goal (which it was not for either FDR or WSC), was some sort of Finlandization of all of Europe. But I think it's very unlikely that Stalin would have accepted that- he demanded buffer space for the USSR, he wanted to loot all the factories etc. of Europe to help the USSR rebuild. He wasn't inclined to trust "oh they'll all be neutral in the future." And the only card that FDR and WSC actually had, as far as I can tell, was to start a massive war while World War Two continued in the Pacific. We know that VJ day followed VE day by only a few months, but no one at Yalta expected that, they fully expected Japan to last until 1946 or maybe even 1947. FDR's most important goal at that conference was to get Stalin to fully commit to war against Japan, he's not going to threaten war with the USSR over Eastern Europe. And absent the threat of war, I don't see any negotiation tactic that FDR and WSC could use to free Hungary. Like I said, if Stalin was feeling surprisingly weak, maybe the West could have traded West Germany+Low Countries+France+Italy being fully demilitarized and forcibly neutral for the same in Eastern Europe, but I don't think it was possible to reach that agreement and I think it would have been a bad trade if they could.

After the war, Churchill blamed FDR for selling Eastern Europe out at Yalta because he wanted to hide the "Percentages Agreement"- which again was purely a UK-USSR agreement with no evidence that FDR ever knew about it- after the Cold War started. But the last German defenders of Budapest surrendered only a day or two after the end of the Yalta conference, Soviet domination of Hungary was locked in long before that meeting.