Not much to report tonight, the boss gave me his laptop to fix :(
I checked out U208 and it's working as expected:
Red values are my working FDIM, black values are the broken one (showing the usual ~0.2v drop while sleeping). Brown values are the regulator outputs that are going through "voltage dividers" to make them work with the chip... 5 x TH inputs are all grounded - so it's simply testing for 0.6v on the 4 test-voltage INputs. Both powered up and sleeping it's within spec. and the PG & !Reset outputs are correct. The watch-dog function isn't being used - it's triggered in both the working and non-working FDIMs.
I find it weird though, the 3 watch-dog pins all have traces - so they are connected to something - but both FDIMs are doing the same thing here.