I have had a reply from Meridian directly:

"Yes you can use the 218 on the output of the G65 to gain the ability to allow MQA decoding and no, this will not degrade the quality from the processor.

Quote
You need to take the Master SpeakerLink output from the G65, into the SpeakerLink input on the 218, then out of this onto your DSP5200’s. You will also need to configure the sources on the 218 so that everything other than Sooloos (SLS) comes in on the SpeakerLink input.
What does it mean about the 218 sources? Is this something to do with MSR+ and selecting the 218 as the source?

How does this effect the MC200 I have connected to the G65 as an input?

Do I need to make my FR the master instead of my FC still?

G65, HD621, 5x DSP5200SE, MC200.