I managed to resolve my issue!
After a weekend of wracking my brains, trying to work out why the device was able to complete an SNMP walk but wasn't able to add the node from the Poller, it hit me this morning... The issue wasn't with the connectivity between the node and additional poller, but the connection between the additional poller and the primary.
I had a look over my APE and realised the local firewall was enabled like we wanted, however the rules were not set for the right ports.
I spent over an hour looking at these rules first to make sure and be certain that they weren't there before creating a new rule for internal devices to allow traffic between ports 17777, 17778, 1433 and 1434.
I clicked ok... Logged back in to my platform, and tried adding my node again...
With bated breath and crossed figures, I hovered my mouse over the test button... I was one click away from relieving my weekend long headache...
Click...
"Test successful"
The sigh of relief was more a cheer of jubilation...
Not a big win, but by no means a small one either... And hopefully this may help someone in the future