Well it seems like you've gone through the first two stages of fixing a HeaterMeter, anger and denial, which just leaves bargaining, depression, and acceptance :-D
If it is just the buttons then that's usually something easy to troubleshoot.
- The #1 cause of buttons not working at all is the ground connection on the 1k resistor next to the LCD1F header on the base board. The pin closest to the 5V side of the header is the ground so first give that a good look over and maybe give it another shot with the soldering iron and see if that fixes it.
- Check continuity from the ATmega BTN pin to the other end of that resistor, and the BTN pin on the header. Should read connected.
- Stack the LCD/Button board on top and power up the HeaterMeter. Check the BTN pin on the Button board, should be 0V.
- Hold down one of the buttons and test the voltage at the BTN pin on the LCD/Button board. Left=0.54V, Up=1.78V, Right=2.72, Down=1.02.
- No voltage change? Signs point back to the 1k resistor back on the Base board, or the soldering on one of the two pin headers' BTN pins.
- Not all buttons show voltage change? Check the corresponding resistor which should read 5V on both ends with no button pressed. Left=8.2k, Up=1.8k, Right=1.2k, Down=3.9k
- Voltage does change appropriately but doesn't do anything? Check the soldering on the BTN pin of the ATmega on the Base board.
- Weird voltage all the time? Power down and check continuity between the BTN pin on the LCD/Button board header and other pins to look for a short (which could either be on the male or female side)