1. The neon bulb is part of a relaxation oscillator that triggers the strobe at intervals. When the capacitor charges up to a voltage where the neon bulb starts conducting, the capacitor discharges through the bulb and the SCR gate. The SCR turns on, triggering the strobe, the neon discharges the capacitor and turns off, and the cycle repeats.
2. The SCR is a device that turns on when a certain amount of current is applied to the gate, and then stays on until the voltage across the SCR drops below a certain threshold. It's like a latching switch.
3. That tube should work fine.
4. You'd have to modify the circuit to sync the strobes.
Use caution: strobe lights use HIGH voltages.