You essentially just have 2 Emerias and there is nothing against having 2 (or more) copies of Emeria. As long as you have 7 Plains, both with trigger. The only thing you can't do here is copy Emeria in the upkeep and expect to get the second trigger that turn. It won't trigger until your next Upkeep.
Well Crystaline Sliver only grants shroud to Slivers that are on the battlefield, if that is what you are meaning.
So yes you can still return Sliver creature cards from your graveyard to the battlefield with Emeria's ability.
So yes you can still return Sliver creature cards from your graveyard to the battlefield with Emeria's ability.