You are severely wrong on so many levels.
Firstly, I never said Yama used his full power. I only said his Zanka no Tachi: North is not an aoe attack, and Yama has also stated that ONLY his zanpakuto is 15M degrees. In other words, besides himself and his zanpakuto, the area around him is not at 15M degrees. Think about it, if it was everyone would have died already. Also, his zanpakuto does not produce flames. If you haven't figured it out by now, then something is wrong. Yama's zanpakuto compresses all his flames just like Ichigo's Zangetsu compresses all his reiatsu. Hence why Yama's zanpakuto looks like a normal burnt blade. It does not produce flames. It does not burn the area around where Yama cuts.
As for the Quincy, you're also wrong. Like I said, the space around Yama is not 15M degrees. If it was, the ground he was standing on and the buildings around him would have caught fire already. Obviously, the Quincy are not affected by the heat.
Finally, you're wrong again. Yes, Yama could have killed Aizen when he was in SS. But in the real world, no. If Yama released Bankai in the real world, the humans would be dying everywhere from the sudden climate change. Also, at that time we don't know if Aizen was immortal or not, because his evolutions were not complete.
Ultimately, you are wrong. Fact is, the current Aizen is immortal. I couldn't care less if he was immortal or not back in the Karakura town arc. That's years ago.