>> But maybe the best way is using the "expanded digest layout” (Android 5.x & 4.4 and lower), containing sender and headline of each single message.
Supported by Android since 4.1, and supported by Aqua two weeks after 4.1 was released at Google I/O that year.
BTW - internally, Android calls it "InboxStyle", but it seems that the design team like fancy names for everything...
What us mere humans know as "that three dots icon", is officially called "the overflow affordance", wow!