Electrical – SCR triggering using DIAC

It's measuring the voltage on the high voltage supply, so any loading will drain the flash battery.

300VDC through 30M\$\Omega\$ is actually quite a large current (\$10\mu\$A), and would represent several mA being continuously sucked from the flash battery when the HV converter is operating.

With separate grounds, there is no guarantee that a pulse generated with respect to the first ground will have any effect on a circuit referenced to the other. There are at least two ways to handle this. The most obvious is to use an optocoupler to transfer the pulse to the SCR. The other, older, technique is to use a pulse transformer.

^{simulate this circuit – Schematic created using CircuitLab}

You will need to research optocouplers, since some are too slow for your application. Also, if your grounds really get to swinging wildly, a given optocoupler has a certain rated isolation voltage which you need to respect. Given that you have not destroyed your scope, your grounds are probably not that far apart, but it's something to think about.

As a side note, are you sure you need a 33 ohm resistor to the gate of your MOSFET? It pulls 150 mA when the 2N2222 is on, and that's a good deal of current. It's entirely possible that you do it to pull up the gate quickly during turn-off, but it still seems excessive.