If you have a direct lightning strike anything could start a fire.
I would rather the surge was contained as much as possible within the PI.
I like my hardwire as once installed it forces your hand to use it with no added effort.
As long as the existing battery is in good shape you can add similar just fine. Just do not mix AGM, wet cell, start battery, GELL etc. with each other. If storing without a float charge it might be better to fully charge and then separate the batteries at the same time you disconnect to prevent discharge.