'''All fixed and working!!! See main post for answer.'''
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'''Update (9-Sep-2018)'''
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Finally managed to spend some time on this project this past weekend. The projector is diagnosed and fixed!! Turns out there was a short circuit on the processor board.
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This was causing the short circuit protection of the PSU to activate and disable its output (ie setting its output to 0 VDC). This answers dot points 4 and 6 above.
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I also worked out that because of the short, the optocoupler responsible for enabling the power supply output wasn’t being turned on. This meant the PSU sat at 10 VDC as in dot point 5. When I manually pulled the optocoupler high the PSU went into full swing and output the 17 VDC correctly. I also successfully loaded it to 1.5A (PSU cable of 3A) without a glitch. After checking the power MOSFETs, voltage references for feedback, and the PWM & power factor correction controllers they all checked out, and gave signals that I was expecting/calculated. So this told me the main PSU is all good.
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Back to the processor board, tracing down a short circuit can be painful. However, thankfully I was able to isolate it quickly to the local power supplies on the processor board. Narrowing down further I managed to find that it was one ceramic decoupling capacitor one of the local power supplies. I am quite surprised that it failed, In my experience, these things rarely fail unless mechanically stress (ie dropping the board/projector). I’ve replaced it and now everything is back up and running. Yaay!
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Picture of bad cap for the amusement of anyone reading.