So the MQA endpoint is not the DSP speaker. Would the MQA endpoint need to know which speaker (5200SE, 7200SE, 8000SE) is playing the audio in order to correctly correct (!)the A2D errors?
I believe so, and presumably this would be achieved through MHR and/or SL (and legacy?) comms.
if full-fat decoded MQA is not to be available unencrypted then only MHR-capable (i.e. proprietary Meridian) products could be MQA end-points for DSPs?
Yes, this is the logical conclusion - it is highly unlikely that M would allow other manufacturers to use the MHR system
we need an entry-level, MQA-capable end-point. The 808/818 are flagships but a (say) replacement AC200 which could take an encoded MQA stream from an MS200/MS600 would surely appeal to DSP5200SE owners, or even any legacy DSP owner?
Agreed, but the AC200 should not need to be replaced - it will simply stream bit perfectly to the MQA/MHR compatible endpoint (such as the replacement for the MS200).
One other possibility, and this is pure speculation, is that the current MS200 and MS600 could be firmware upgraded to give decoded MQA on the analogue outputs only - this would at least allow some full fat MQA functionality on these machines with, for example, headphones or analogue systems (and would be a good PR move too). I understand that going the whole hog and making these machines both MQA and MHR compatible via firmware upgrade is not possible because of chip limitations (unconfirmed but believed to be true).