Niko Home Control Energy - Niko Home Control Starter Kit switches electrical devices ON in the event of excess injection power to the electricity grid. Will these devices also be switched off if the sun is no longer shining?

Yes, the solar mode ensures that if there is more than a certain power yield during a certain period, a first device is switched on.
If the yield goes below a certain threshold, the device will switch back off after the set minimum duration.  You can set this "minimum duration" parameter per device. This means you can ensure, for example, that your dishwasher keeps running for at least a one hour cycle.