Usable capacity vs lifespan
The lifespan of lithium batteries including LiFePO4 is mainly affected by temperature, depth-of-discharge and storage state-of-charge. While the maximum voltage EarthX lists is 14.6v (~100% SOC at 25C), this is not the ideal charge/storage voltage for long lifespan.
The datasheet provides a few lifespan examples:
4000 cycles with 20% DOD @ 1C discharge, 25C
2000 cycles with 80% DOD @ 10C discharge, 25C
In the ideal case, the charge voltage would be regulated such that the current into the battery drops to zero between 80-90% SOC with charging disabled entirely if the battery is at or below 0C. This would require temperature sensing, coulomb counting, and a battery model to accurately estimate SOC.
My suggestions:
1.) Drop the maximum regulated alternator voltage to 13.8v which should correspond to ~80% SOC at 25C, and select a battery model sized to handle emergency landing with all desired systems active in the case of an alternator failure, accounting for the 20% reduction in capacity.
2.) Buy the vented style and keep it in the cabin. If the temperature is ever at or below zero, heat the battery/cabin before starting the engine or before enabling battery charging.
Here's an article with more detailed info.
https://batteryuniversity.com/index.php/learn/article/how_to_prolong_lithium_based_batteries