"Re:Could the R&A and the USGA be merged?"
In effect, the similar things the R&A and USGA are responsible for in their separate regions are far more unified now than they've ever been, mostly in the playing Rules of Golf and in the I&B Rules and Regs of golf.
Would they ever actually merge into one world-wide organization? I'd say never and for a few reasons---
First, they have their own national and organizational histories, tradtions and pride about various things that would prevent that.
Second, it's a liability thing to both of them---eg they fear they would be sued by manufacturers on the grounds of monopoly constituting restraint of trade.
Third, it's a money thing. The R&A would have to contribute way more than they'd be willing to or probably could afford in order to maintain the same level of authority they have now and have had.