Check if electrical current exists in high voltage
Andy already showed you how to connect 2 switches to 1 load. But this isn't the best solution for a home automation system. Better is to have all inputs (switches and buttons) connected to the Raspberry Pi, and at the other end have the actuators (relays and dimmers, for instance). Separate inputs from outputs.
You mention more complicated wiring, but on the other hand all your inputs can be low-voltage.

As for the number of relays you can connect, there's no limit. Use cheap 74HC595 shift registers and daisy-chain as many as you need. You'll only need 3 lines to control the chain: data, clock and latch. You clock the successive states of the 1000 or so relays into the shift registers, and complete the action by applying the latch signal. 

To detect current the current transformer you mention is one option, but there's also the Hall effect sensor, which detects the magnetic field caused by the current and is most often an IC. the Allegro ACS714 has an analog output, so that you not only can detect whether current is present, but also how high the current is. Costs much less than the current transformer.