Definitely the XL engine. The other systems are neat, but the greatest impact on survivability would be the XL engine that can survive the destruction of side torso.
Yes, but the aim was to run it at lower cost, so it's standard ammo. Lasers allow for some sniping even when the LRMs are gone as clanners don't want to retreat to rearm in general.
You can't apply them to the 'Mech you are creating. At best you can just write them in the fluff, but that's it at least for now. Maybe Mordel will apply them at some point.
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