Don't feel too bad, modern ground schemes can be fairly complicated and often require complete re-assembly to run properly.
  So it's all done & ready for use now? 

No. 
This is with nothing plugged into the input?  Do volumes have any effect?
Plug something into FX return jack, still same problem on turn on?

  The connectors are just push-in.  Try not to pull by the wires.  At each end there are little lip edges that you may be able to get your fingernails under.  Or a flat head screwdriver to pry each end up a little till it comes loose.

In you first post you had your +15V.  Now it's gone.  Either something has gone bad or there is a connection issue.  Check at the output pin of the 7815 regulator, there should be +15V there.  If it is not, either the regulator is bad or something is loading down the +15V line, like a shorted IC.  Or the regulator is not receiving it's input voltage (probably around +20V or more).
  If there is +15V at the regulator output, then it is probably a connector issue or a bad solder joint or cracked trace somewhere.  In this case, check all the IC's for +15V, if some have it and some don't, it may help localize the problem.

  Also, have you tried contacting the designer and asking him about it?  He sounds like a pretty helpful individual and may have some suggestions.

  Nothing in that picture looks non-repairable, so I don't know why he would want to replace the board rather than fix it.  Perhaps he does not really repair things at component level, in which case you will need a different tech.
  Suggest you contact this fellow to get a copy of the schematics:
http://rolandtd9review.com/simmons-da200s-schematics

  Also on some marshall models you release the standoff from the outside of the chassis.  Sometimes by screw, sometimes by 2-piece black plastic push-pin.

  Amen to that!  For me that pretty much describes the state of modern Marshall schematics.  Occasionally someone online will make up a drawing showing which connectors go to what.  Otherwise they are a nightmare.  I often wonder if the actual factory techs have secret maps of the connector routing.  If they still have such a thing as factory techs....

Hawk:  D11 is part of the supply feeding the 7815 regulator.  As you mentioned you are getting +15V to the op amps, it is not causing much problem at idle.  But perhaps when passing signal the +15V would be dropping and making trouble.
 

   Seeing as this is your "test mule", you should be warned about the 7293 output IC's.
  At any time they are disconnected, before re-connection their power supplies must be discharged.  If not, they can be damaged.  So, if you disconnect their board connectors, you must discharge their supply caps before reconnection.

  Also note the frequency response.  The 8MRA is designed as a mid-range driver with freq. response dropping off below 400hz. 
  This may or may not be an issue, depending on how much low end response you need.
  But, keep in mind, A440 is the high E string at the 5th fret, open low E string is 82.41Hz.   

  The approx. 30V on both sides of that resistor is good. 
I outlined earlier how both channels are separate until C32 where they join.  As the problem occurs in both channels, it is then likely the fault is after C32.  You said the fault also showed up at the FX send jack, so it should be between C32 and FX send.  If it's a bad connection, it should respond to physical input like tapping the circuit board with an insulated item such as a plastic pen body or chopstick.
  If it's an actual bad component, the most likely suspects in that area are TR12 and C33.

  Yes it needs to be on. 
But before you even do that, measure it's resistance, meter set to ohms, one probe to each side of resistor.  It should measure 220ohms or less.  If it measures much more resistance than 220ohms it is bad.

  No you can't run both outputs into the dummy load as there are 2 separate power amps (stereo).
But what are you trying to accomplish?  Dummy loads are only required for tube amps.
You can run a solid state amp happily with no load.

  Then if both channels are the same, it must be something common to both channels.
That leaves the power supply to the preamp, or the circuitry between C32 and the preamp out jack.
  What are your DC voltages on either side of R20 ?

schematic attached
Is the problem the same for both channels?  Do all the controls seem to be doing something?

  The amplifier power will be split between whichever speakers you use.  If you plugged it in to a 4x12 cab, the 120 watts would split 4 ways.
  With 2 speakers, they will each get 60 watts.  A bit of a safety margin helps, you might consider two 75watt speakers if you play loud.

But just as important is the speaker impedance.  The amp is designed for a 4 ohm minimum load.  So you would want two 8 ohm speakers wired in parallel to make a 4 ohm load.  That is probably what the stock speakers are.
  Two 4 ohm speakers wired parallel would make a 2 ohm load which would overheat and damage the amp.  They could be wired in series and make an 8 ohm load, which would be safe for the amp, but the amp will put out a little less power into 8 ohms than it will into 4 ohms.