No, era scaling itself isn't affected. However, instant yields also scale with game speed (but this is not written usually) with the same factors as all costs scale (1 for standard, 1.5 for epic, for the others I'm not sure (Marathon has 3, I think, and quick 0.67, if I recall right)). AI bonus yields from difficulty generally use another scaling with era. You don't get more happiness, no. And if the civ that wants to buy a second copy is Netherlands, everything is fine. Since a few patches ago, Netherlands is allowed to buy multiple copies of luxuries and can obtain monopolies through this (and trigger his UA). Other civs shouldn't be able to do this, if I didn't miss something.