With the data-center changes coming up, EU will have a rather glaring problem:

50% of the player population will disappear overnight.

Now this might be fine right as the new expansion launches, but it will create issues 2-3 months later.

It will make it a lot harder to do older/less popular content, it will increase queue times, and overall it will increase player frustration.

Has SE promised any data-center merges or compensation for all the players affected?

I doubt they'd just split an already smaller in half and just assume that the players would be fine losing 50% of the people they could group with.