So a quick hot-on-the-heels update:

1. Satnav entirely out of the blue gave me camera warnings during my trip today - and without being restarted since the last trip last week. It was connected via BT to my phone also.
It's at odds with the previous behaviour when it was consistently silent and had to be powered down for the warnings to reappear.

I do wonder if the chap @ Garmin has done anything his end during my chat with him last week though, because...

2. ...because I just found out in the satnav's Smartlink app menu - never ventured there before really as I was never interested in any of these 'live' features - that real-time safety cameras warnings (via BT) are proudly present there (and also shown as FoC...).
Again, it's at odds with what Garmin advises on their web pages, but I did see inconsistences before so if this unit is not listed there, that doesn't mean it would not support this feature.

So it would appear that safety camera warnings SHOULD work for me 100% with BT enabled & Smartllink connectivity in place.
It still does not tell me anything about PoI (GPI) files - although if they're seen by the device, they also should work all the time.

As the results are not exactly conclusive I will carry on with my tests, with and without BT enabled...