Every plugpoint at home contains a neutral, but what happens or how the return path is closed when the neutral of the power station is not earthed?

with out neutral,y connot complete ckt.

Yes neutral is necessary while we are using 3 phases.but in some cases we can use 2phases i.e. one phase as incoming and another phase as return path.

neutral is necesssary because the current flows in a close circuit and if the there is no close path then circuit will not complete and no current will flow ...

neutral means point of zero potential. in a 3-phase power system when 3-phases are star connected , that star point is called neutral.but the potential of neutral point is not zero if the system is unbalanced. to make the potential of star point (neutral) zero , that point is earthed so that even if the system is unbalanced the potential of star point remain zero.

why neutral point is usually at the common connection on the secondary side of delta/wye connected transformers.

Why is neutral called so?

A circuit needs a reference point to work well