Leaving this here for anyone struggling with assembly of the two arm parts.
I was having huge issues assembling two of the arms together, the other two went together relatively easily. Furthermore, the idler gear was a really snug fit to the driven gear on these arms.
After spending a fair bit time diagnosing the difference, I realised that there is a limited amount of play in the position of the motor (relative to the face of the moulding).
Loosening the machine screws and holding the motor towards the RJ45 jack and re-tightening fixed these issues for me. The idler gear went in smoothly and the two halves went together much easier.
Hope this helps anyone having similar issues.