Current carrying capacity of nickel plated plastic?

Electroplated nickel has a specific resistance of about 8-9 micro-ohm-cm. electroless nickel phosphorous ranges from 30 to 90, Tin-lead solder is 13-16, gold is about plated gold is about 3-4
Yes nickel is conductive.

Nickel is a metal so it will conduct electricity. However, very thin layers of electroless nickel are poor conductors because they often include a lot of oxide and are discontinuous. It is very common to put down a couple of microns of nickel and then thicken the metal layer electrolytically, but it is important that the metal layer is adequately conducting, otherwise you will burn the layer and create a high resistance area.

You can use conductivity to measure the nickel thickness but it will only be a guideline as the composition of the metallic layer will also influence its conductivity.

Electroless nickel baths also have a tendency to vary in performance as they age, so it is difficult to keep and maintain a "standard electroless bath" without regularly remaking it or at the very least carefully replenish it