I've made some progress in answering my first question, having now found http://pinout.xyz/pinout/scroll_phat which indicates that the Scroll pHAT uses pins 3 and 5 (presumably for I2C).
However, that doesn't account for power. I don't know on which pin it expects to draw (5V?) power, and where it expects to be connected to ground - can anyone help with that? While it seems that at the RPi side the 5V pins are identical and the ground pins are identical, it may be that at the Scroll pHAT side it is using connections to particular ones of these pins, and I don't know which.
Plus I'm still keen to know whether my option 1 or option 2 (or some other option) is feasible for being able to access other pins!