Having time in the B767 as a captain I would agree with their decision to continue to their base and burn off fuel. It made the landing at a lower speed and lots of time to comunicate with their company. Why the alternate landing gear extention didn't work is still unknown.
Landing early in a situation like this to avoid a crossing of the Atlantic probably wouldn't have helped matters. Yes, possibility of further failures due to the loss of Center hydraulics must be considered but as I recall the center hydraulic system can be isolated and have no direct connection to the engines. I haven't flown one in 8 years because I turned 60 so don't remember.
As I recall it is all electric pumps and pnuematic air pressure pressurizing the center system. They did a great job of sliding it in and don't think they could have done anything more to improve the outcome.