Just as a result of the day job and multiples of experiences in small, medium and very large scale IP Installations
The simplest answer to your first question is that your Router (certainly the commercially available ones aimed at home users) will really "actively manage" the Devices within it IP Address range.......more capable (therefore slightly more expensive) Routers will get around this, but the HomeHub is not one of these I suspect
It's of some consequence that Anthony has only been experiencing these problems since he updated to 2.355 on Monday
By contrast, I've used ControlMac on three Mac's without issue since Monday
Whatever BT are doing with the "different" settings on different "ethernet ports" is also of significance here I strongly suspect