So after half a weekend of playing with Houdini Engine in Unreal what I've found is most errors can be sourced to a linkage issue which I solved by doing the following
1 - remove any existing instances in your World Outliner
2- select HDA in your Content Browser>Right Click> Find Source
-if it correctly links to original HDA on your desktop great! Reinstance and try again. If not proceed to next step
3- Link with original HDA. Select HDA in your Content Browser>Right Click> Reimport
4- Reinstance (drag to viewport) and try again

The second issue is when there are no errors in your Output Log but the hda is not updating even when the linkage is correct, ie there is only the 3D Houdini icon even after doing a Rebuild.

If anyone can help me with this latter issue and how to get the 'Sync Viewport' feature working it would be greatly appreciated!