Well, if you want to listen to the same music through the speakers that are connected to the Maranz today as on the other HEOS speakers and also want to upgrade to 4k and/or need the 13 outputs of this AV receiver, it might make sense. However, only you can answer whether this is the case :) But I hope this helps you with your decision.

According to HEOS development the following answer: at the time of the evaluation (Nov16) the feature "TV Sound Grouping" did not yet exist. This ensures that the delay between AV sources and connected rooms is in a range where it is negligible. "TV Sound Grouping was added to the HEOS Core AVRs in April 2017. It is switched on in the HEOS app under Settings/MyDevices/AVR-X...H/TVSoundGrouping. So no problem (anymore)

This is a matter of principle: Heos is a digital streaming technology, which involves delays - the receiver would have to buffer the picture and also delay it so that it fits (read up on whether it does this). Streaming: signal buffering->transmission->transmission time->reception buffering->output

Yes, Dolby Atoms speakers fit. Use this filter to find the current range: https://www.digitec.ch/de/s1/producttype/hifi-heimkino-lautsprecher-51?pdo=91-9747:287740&tagIds=538-588