Sorry EASA certification of an aircraft is enough for the FAA. The EW aircraft had to turn around because the complex wetlease deal SN/EW had set up was not approved by the DOT. More on this mess is here: https://www.ch-aviation.com/portal/n...-blames-france

New Airbii can fly to the US pretty much instantly. Any modifications done to EASA certified aircraft need to be approved by the FAA if its operated under the US flag. Delta does this with all new Airbus deliveries.
If SAS is doing some retrofits in CPH on SE-REH (not everything in the cabin and flight deck is line fit in TLS/XFW) then the Danish regulator has to approve the work. Every new aircraft has this done so there should be some routine for that. Danish approval is in line with EASA and accepted by the FAA or for that matter any other country SAS flies to.