Questions about reflowing solder on a motherboard
Heatgun with aluminum foil warped around the rest of the board loosely, to act as a heat shield. You only want to heat the bad chip. No need to reflow everything else. If there is anything on the other side of the board, that might be an issue. This is just based on what I've read, no personal experience on this.

The reason that the chips don't die when baked at a high temperature is because the temperature is external, and takes a while to transfer through the ic package. Not only is the package designed for this in the first place (SMD/SMT was created for both size and ease of machine automatization. It allows for simple reflow after using a pick and place machine, instead of requiring manual soldering), the reason high temperatures kill ics is because the internal junction temperature is too high. Without proper cooling, the inside cooks itself. It's the reverse of normal baking.