Why bother with the AR-10, it's about 150mm longer than the EM-2 and yet has a barrel about 100mm shorter. No, if they can get the .280 in, then the EM-2 is coming along as baggage, and it's a more compact weapon, and comes on the scene 5 years earlier too.
Because not all armies like bullpups. I would imagine most Commonwealth countries would end up using the EM2 and most European countries would go for a 7mm x 43mm FAL, though there'd be "defectors" on both sides. America might flirt with a .280 M14 but if the EM2 sees a bit of action in Korea they're more likely to opt for a .280 AR10, which raises the amusing idea of a .308 AR15 appearing in the 60's. Unless the Belgians change their mind about giving the Germans a licence to make FAL's the HK G3 would be in 7mm NATO, as would Spain's CETME and Italy's Beretta.
France and Switzerland are interesting though. Would they make the change to a NATO calibre or stick with 7.5mm? By the time the EM2 has been accepted France would already be using the MAS 49, perhaps though the French would change to the 7mm round in the late 50's when they were looking at more modern rifle designs?