On this theme, I also wonder about the level of support this problem is being given over the weekend. My guess is as good as yours, and likely the same.
On Friday, Support mentioned to me, more than once, something about an IE 10 update between 4/18-4/19. I failed to find any evidence of such a change on my PC. That doesn't mean MS couldn't have made a change and hidden it from me, but I haven't seen that kind of action from them in a long time.
Per my PC's log via Control Panel-Updates-View Installed Updates the last time IE10 was touched was 10 days (4/9/2013) before this latest FTM problem appeared on 4/19/2013. My experience suggests MS isn't to blame in this instance (I can't believe I even said that).
After losing 2 working days due to this issue I tried the method others had suggested of reverting to IE 9 (my PC is Win7 so I had that option). That restored my ability to Merge and I ceased chasing the root cause of the problem.
This has certainly been a frustrating issue and IMHO a solution should have been available in hours, not days.