1) Correct, you can apply regulated 5V to +5V (aka VIN) and GND on the terminal block. You can do so simultaneously with 24VAC transformer plugged into the orange terminal block.
Sometimes people are worried about two 5V sources (OpenSprinkler has an internal 24VAC to 5VDC converter) fighting with each other. This is usually not a problem because both are regulated, so at any time, the one that has slightly higher voltage will take over. But, if you want to absolutely avoid having two 5V sources at the same time, you can do the following: on OpenSprinkler v3, the leftmost pin of the orange terminal block is internally wired to COM (common). You can remove that wire from the terminal block, and also remove the COM (common) wire, then connect these two wires directly together, without going through any terminal block pin. This way, 24VAC will only power the solenoids (and NOT generating 5V via the circuit), and your external 5V will provide the logic level voltage to the controller. This will avoid two 5V sources competing with each other.
2) On DC-powered OpenSprinkler, correct, you can feed 12VDC directly to the DC power barrel (the polarity is center positive, side negative, which is the most common type). No need to mess with the 5V/VIN pin.