1) Can transmitter for Canon trigger receiver for Nikon and vice versa?

Answer is yes! They work on the same frequency and as long as you're in the same channel, they'll work!

2) Can a Nikon flash be put onto the Canon receiver and vice versa?

Yes! The flash on a receiver is basically triggered by the center pin and this flash will have to be in manual mode (NOT master nor slave!)

3) 2nd curtain sync of the receiver

Unforunately, this won't work, if the main flash on the transmitter is in 2nd curtain mode, the receiver will always be on the first sync. One way to have remote flash and yet have the 2nd curtain is to use the Nikon CLS system, and master/slave flashes all need to be in 2nd curtain mode.