Currents split inversely proportionally to the resistance of the load when in parallel. When the loads are in series, the current is the same in each step of the series.
By superposition, yes, all the parallel currents add up.
As long as the voltage compliance of the source with the narrowest range is not exceeded.
EDIT: I actually did an experiment with a pair of JFETs. As long as the supply voltage was high enough to operate both devices in saturation, the resulting current was IDSS1 + IDSS2.