Hi Jim Zheng
Welcome to the EK forum. It's good to see you here and we look forward to hearing about your EK experience and as you can see, there is a wealth of knowledge relating to EK here.
To your specific question, according to the seat maps on the EK website for all versions of the A380, row 51 only has the centre block (D,E,F,G) and if by my experience of only one Y flight on an A380, and they are all the same, then there is a wide walk through space between these seats and the galley so should recline fully. Also, on the EK Source website, unlike other rear bulkhead seats, these seats are marked as regular economy seats. The two immediate drawbacks I can see are 1: they are in economy and 2: they are right in front of the galley, so galley noise could be a downside.
Hopefully someone one with more experience of Y on the A380 will be able to confirm.
Once again, welcome onboard and hope to see more of you.
Safe & Happy Travels
S