That voltage spiking to 100V and then down is probably a combination of the Leaf RLY N/P and precharge relay switching in a certain order every time the pack is connected/disconnected. This guy has the sequences on video:
So that is nominal, nothing particularly wrong I don't think.
Be very careful when opening (and more importantly: closing) the main battery. It's gasket sealed all around for a good reason. At least one person had his Leaf flame out due to water getting into the main pack. Caulking is not enough, you need a compliant gasket around the pack to properly weatherseal it.