While Atteact Mode undergoes further improvements with a release of v.2.5.1, the really big news of this weekend is that a forum user finally found the right settings under Windows 10, and this immediately resolved my problems in MacOS High Sierra as well:
The only thing I had to change was in the Emulator settings - specifically for MAME:
Non-blocking wait mode - change from 0 to 1
That's all. Now MAME correctly exits back to Attract Mode on my machine just as before. No hanging black screens and force-quit of AM any longer. Very good - kudos to user mrteeney!