1. Does the built in head unit of the LD400Pro have the same exact electronics & power output as the separate HD400 head unit (both have the identical preamp & power amp sections)?
A: The preamp is the same, the power amp is a bit different due to the internal speakers.

2. Does the LD400 put out the full 400 watts to its 2 X 10" speakers?
A: yes

3. Do the internal 2X10 speakers have 4 ohms or 8 ohms of impedance?
A: the internal speakers are 8 ohms

4. Is the 'Extension Speaker Output' in parallel or in seriers to the 2X10 internal speakers of the unit?
A: parallel

5. When an external cabinet is connected (such as your LowDown 410 Cab at 8 ohms), will both the internal & external speakers be driven equally?  At what wattage?
A: The external cabinet may be a bit "louder" due the ohm rating and more speakers being used, wattage will depend on the ohm rating.

6. If I were to start out now with just the LD400Pro for practice and small gigs and then later found it necessary to add an external cabinet for more volume to handle larger gigs and outdoor events...and I connected a product like your LowDown 410 Cab, how well will the power amp section of the LD400Pro be able to drive this new array (now a 6X10 setup)?   Will it be very underpowered to the point that a more robust solution should be considered instead?
A: You should be fine for most applications running an external cabinet, but you will need to decide for yourself if you will have the power you need for your specific application.