First of all, yes you can register with Skywards before flying. Just go to www.skywards.com and click on join. Once you did this go to the emirates page, go to manage my booking and then you can add the Skywards numbers for all travellers that have one.

Regarding the seating on the A380, row 51 only has seats DEFG. For ABC, HJK row 50 is the last one and there are no seats in 51. It is true that 50 ABC, HJK only have limited recline so you might want to change this.

One last thing, Seatguru is pretty outdated for some aircraft types so you should always stick to the seat map emirates gives you for your flight.