I think that the color of the FET heatsink tab tells all. It indicates that a lot of current was passing through that device i.e. lot's of heat. Since there are three FETs in parallel to share the current, it is obvious that that side was not sharing. Two things are possible: 1) either the device was defective and started to produce a lot of internal heat or 2) the other transistors were not pulling their weight. They may have gone open or there is another failure on the board.
There is always an outside chance that a stray wire came in contact with the tab on that FET. It happens more often that you think. Examine the tab closely for signs of a high current entry point. It will be rough in texture and exhibit more discoloration than the rest of the tab.
I wanted to speak to the no load voltage measurement for batteries. One must pay homage to Ohm' Law when making measurements with modern equipment. A typical Fluke meter has an internal impedance in the Megohm range. Ohm's Law solved for voltage drop with that kind of load, will produce an imperceptible voltage drop across the internal impedance of the battery. (what is the current supplied by the battery into a 1 Megohm load?) Hence, even a depleted battery when cool can measure nearly full voltage with that type of meter. Add to that, the varying resistance of the connection of the meter probes to battery introduce significant error even in fully charged batteries. I have checked batteries from various teams that show good when fully charged but have one or more cells that have a reduced capacity after delivering current for a few minutes. This indicates to me that a plate(s) internal to that cell have become disconnected and therefore no longer supply current under load. The CBA II battery analyzer from West Mountain Radio is a simple device to check batteries under constant load (about 7.5 amps) and performs a nice plot of time versus terminal voltage. The data can be stored for each battery, recalled and compared with the subsequent tests of the same battery over a period of years. Data can be overlayed for several batteries as well. West Mountain also has a higher current addition to their line of products but we have not purchased one yet.
http://www.westmountainradio.com/CBA_ham.htm