Back on the original issue of the extra notifications and random max-volume of some alerts I’ve just had twin SMS messages arrive; I got 2 generic notification audio (pre-)alerts, (again at maximum volume), then got the 2 actual SMS alerts with the custom notification sound (at the normal volume) for that sender. I also caught the first 2 alerts message actually appearing on the lock screen briefly (but couldn’t see the unique app name or icon, just some white text on black background, with something about ‘notification of message’ or similar) - but when I tapped on them and unlocked the phone they were no longer there (i.e. visible). And there is nothing in the notification history for them - just the entry for the SMS Messages app notifications.
So 4 sounds (2 at maxed out volume), and 4 notifications appeared ‘on screen’ (if only briefly), but only 2 entries in notification history.
P.S. also found a couple of apps that appear to have additional notifications, at least that I hadn’t come across previously pre-7.7? And they have audio notifications enabled by default… (e.g. ioDroid had audio notification enabled for when it self-updated - it was supposedly an unused notification but might explain some random alert sounds I’ve been getting - at odd times - with no apparent cause or notification message)?