Nickel Metal Hydride is a safe, non-volatile chemistry, and it is inherently safe. Lithium ion cells contain far more energy, and lithium itself is volatile, can spontaneously combust in air on exposure to oxygen, nitrogen, and/or carbon dioxide, and it will explode if exposed to water (to be clear, raw elemental Li will explode if exposed to water, but I wouldn't expect that to happen with an intact Li-ion cell, only that it would lose capacity and if exposed long enough to water or moisture, rust could form, and it is possible the cell could short itself, which would be bad).
Eneloop Pro are more fragile and less tolerant to abuse than standard Eneloop. And even under ideal conditions, after 50 recharges, both the Eneloop Pro and standard Eneloop will have the same charge capacity, though the Eneloop Pro is only rated for a further 450 recharges, though standard Eneloop for a further 1600+ recharges.
The idea for dedicating cell groups and swapping cell positions at each use is to give the cells even wear. Splitting up your cell sets necessarily means the individual cells will have different experience and will no longer be closely matched, meaning in a two cell light one of the cells would probably be discharged faster than the other, and it increases the risk of one of the cells reversing polarity, but to no ill effect other than being left in the dark. There is no danger with NiMH, only that your cells may degrade, or one may be in a more degraded state than the other, and the performance of the light will follow the performance of the cells. But, again, nothing bad will happen with NiMH no matter what you do them, unless that be to drop them down the barrel of a shotgun. That could kill someone when discharged.
Not really, it's because of performance. Water and vapor will cause the cells to self-discharge at an increased rate, resulting in reduced capacity, but water can also damage and possibly short the cells. You want to keep your cells dry.
The amount of gas that possibly may (or may never) vent is insignificant, so even in the off chance the cell vents, if somehow the gas didn't dissipate, which it would, and then somehow it ignited, even if it ignited next to your face there could be no damage, due to the insignificant quality of gas.
The increased pressure would be insignificant, only enough to bulge the switch boot slightly, and I don't think there would be any way to ignite the tiny amount of gas unless you were already in fire in a room on fire, and even then it wouldn't do any damage. It'd be the already being on fire part you'd be worried about, not a little pop from a tiny amount of hydrogen gas.