Well, one would assume the use of the UHD722 with a Meridian surround sound processor, in that scenario you should be fine with everything. ARC allows 1 SPDIF's worth of data, so that could be a lossless CD or HD stereo track, or it could be Dolby Digital or DTS (plain versions, not TrueHD/Master Audio). These would simply be routed to your M processor which could handle them all.

If the UHD722 supports eARC (I don't think it does but maybe?) then it wouldn't provide any support for Atmos or dts:x, but then I imagine the UHD722 would advertise that it doesn't support those protocols anyway. It could however receive 8 channels of PCM which it could feed to an 861 or G65 (the latter would be able to accept 6 of those channels).

Lounge: LG 55B6v, Some Denon receiver, Anthony Gallo Reference AV mains, Totem Dreamcatcher centre, SVS PC-4000, AppleTV4K
Office: Audioquest Dragonfly, Cyrus 6DAC, Monitor Audio CPW Gold
Headphones: NuForce HDP, Audeze LCD-2, Sennheiser HD6XX, B&O H6mk2