Mega 2560 & smoking/burning 5v requlator, probable cause?
They are using the same SOT223 LDO as every other product.  These have a very low limit of output current when run from an input voltage such as 12V.  If you replace the LDO with a switching regulator, you can eliminate the waste heat regardless of the output current (Except for the 5V to 3.3V secondary LDO which is a constant).  The PDF linked below shows a comparison between the LDO and drop in switcher.  Not that the switcher performance at 1900mA is equivalent to the LDO at 250mA... at 250mA the switcher temperature rise would only be 8.5C.

https://www.sugarsync.com/pf/D3258881_07979653_7526812