Sorry to hear of these troubles - I have a 5000 system and I love it.
It is good that the Beogram and CD55000 are working on the Marantz.
Are you able to test the Redlines the same way by connecting them to the Marantz?
When you supply power to the Beomaster 5000, you say its red standby light comes on ... do you also hear some clicks from inside? does the unit generate a small amount of heat at the back?
Are you able to press the bottom left hand side of the mental front piece of the Beomaster (it is marked "OPEN") and odes the strip open by folding down?
If the strip does not fold down, try gently opening by using a finger nail to apply pressure so it swings open.
Now see if pressing any of the revealed buttons cause any status lights to come on.
Make sure you fresh batteries in the MCP 5000 and that those contacts are clean.
Finally you might try using a mobile video camera to see if you can see any transmissions from the MCP to the Beomaster.
Hopefully this will be easily fixede, the good news being that major parts of the system are working on the Marantz !
First B&O (1976) was a Beogram 1500 ... latest (2011) change has been to couple the BL11 with the BL6Ks *sounds superb*