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Try Nick Gammon's actual circuit.
Start by taking off the regulator and all the LEDs (or their current limiting resistors) with the tip of a solder pencil.
It looks like the Pro Mini has a jumper to isolate the regulator (and the power LED). If you can identify that jumper and de-jumper it then you wouldn't have to remove the regulator.
The other LED you could avoid touching by making sure that pin is set to either tri-state with no pull-up resistor or output low.
I've been disappointed that I could only get my processor down to 5uA in power-save mode.
The regulator and LED consume a heap! Don't even bother trying to save microamps while you have an LED draining 2 mA all the time. You won't notice the difference.
The output of the regulator is the one on the lower right of your last photo, the right "leg" of the two pin side. You should be able to follow the trace to pin 4 (Vcc) of the 328. Either cut the trace right next to the regulator pin or lift the pin with an exacto knife while it's molten from the soldering iron.
I'm not sure about that gold thing. It could be a polyfuse.
You're right, I didn't read it carefully.
I think your method should work. With enable low the regulator couldn't be drawing more than 1uA, probably a lot less. The datasheet will tell you.
Or add that jumper to the output that your board seems to be lacking.