There are two functions that these "redundant" pins provide. Using your first datasheet as a reference, the grouping of pins helps to show their function.
Groups (9,10) (13,14) (15,16) and (11,12) are tied together to reduce resistance and inductance.
The pins directly connected to the source of each transistor (1,3,6,8) are lower resistance, lower inductance connections to the sources which is better for the gate drive circuit. Keep in mind that some gate drivers can drive over 9A peak gate current, so minimizing inductance is a must.
In terms of tying control pins together, that isn't possible with power devices due to dead time switching requirements and gate drive voltages.
The prognosis is probably not too flash [groan].
Depending on energy of radiation, device death ranges from possible to essentially certain.
You can potentially expect all the effects you described.
Even if apparently still operating you may get increased leakage currents and 'general unhappiness'.
What was the gamma source? what energy?, what distance? Why?
Radiation Damage in Electronic Memory Devices PDF
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Says:
... Obtained results show that gamma radiation causes decrease in threshold voltage, being proportional to the absorbed dose of radiation.
EPROM & EEPROM: ... Gamma radiation causes generation of electron-hole pairs in SiO2 insulator of the gate. The number of generated pairs is directly proportional to the energy deposited in material, depending on the total absorbed dose of radiation [8, 14]. ...
Conclusion
... Based on analysis of data gathered from performed experiments, the exposure of semiconductor memories to gamma radiation causes three effects: holes being captured in trapping sites of an oxide, injection of holes from oxide into FG, and emission of electrons through FG-oxide interface.
The generation of electron-hole pairs leads to trapping of positive-charged carriers (holes) in insulator, causing negative shift in characteristics. Namely, positive-charged carriers induced by gamma radiation require the increase of negative gate voltage to compensate the positive charge. It means that gamma radiation causes decrease in threshold voltage, being proportional to the absorbed dose of radiation.
NASA - Chips in space
Best Answer
Taking the example of the chip in your picture (RHFAC14A - datasheet,) they aren't always in the flat package with long leads (FPC-14.) They are also available in DILC-14, which looks more like a standard DIP-14.
So, the flat, long leaded style isn't a requirement of the radiation hardening.
Wikipedia says that type of package is known as a "flatpack" and that it is a package specified for the United States military.
And, there's your explanation for why radiation hardened parts often come in flatpacks: The US military is one of the larger (if not largest) purchasers of radiation hardened parts. You get a lot of parts made to meet that standard just because the US military is one of the biggest customers for that kind of thing.
As well, they've been making the "flatpacks" since 1962. Those are surface mounted parts that have been around longer than there was any real market for surface mounted parts. Anybody who has been building small devices since before SMDs became a standard thing would have likely had to use the flatpack devices if they needed surface mount style parts.