a - yes, but it was rectified in further emails
b - you will receive service according to the fare class on your e-ticket (attached in the email, as the e-ticket forms the contract), not the screenshot confirmation. If the e-ticket confirms you in First, aka a fare with basis F,A,Z, you could then have a case to receive F service, but EK are also within their rights to give you a full refund and rebook you into economy

If you bring it up at the airport, you will be issued what is in their system (likely economy as you say) and then if you try and turn that into an upgrade, you'll be directed to ticketing/duty manager and possibly may miss your flight, or have your ticket refunded, if you don't wish to travel.