Great dilemma isn't it...this hobby definitely has its challenges! The answer to your first question about the channel configurations is yes, you can use the amps in the receiver to handle the Atmos channels while your external amp is running the main 7. The only amplifier channel not reassign-able in 740 is the Center Channel. Also, the sub outs are independent and not summed to mono as before - this will be an upgrade in ARC Genesis' Software in managing those.
There is a sonic advantage to having a straight Pre/Pro versus using a Receiver - my opinion (so I don't want to offend anyone), to which, I think you'd be better off using the AVM70 and just running 7 channels versus trying to add Atmos Speakers and using the receiver as the Pre/Pro, especially if your amp has balanced inputs. The AVM70 uses a toroidal power transformer with lower noise than the iron core transformer used in the 740. See "Truly Balanced" vs "Fully Balanced" :
https://www.anthemav.com/support/faq.php
We're assuming your amp is better than what's in the receiver and that's the whole purpose for proposing a Pre/Pro over using a Receiver to begin with. 7 Great Speakers, or 11 or 15 just ok/mediocre speakers? It's a quality over quantity debate, but I think you highlighted all the features that the AVM70 has over the 740 except that all of our Receivers and Processors are supposed to have an HDMI 2.1 upgrade path.
Once they're all shipping and available, it should be easier to get a grasp on everything. Thanks for the questions and have a Happy New Year!