So I guess it's just me but it seems that there's so much over complication going on here about Luke's credibility. It's very simple and already in the original presentation of the Star Wars film.

Ben Kenobi mentions Luke's Father being a good pilot. We can assume he's a good pilot too. Luke goes on the journey to rescue Princess Leia. Leia knows Ben, goes with Luke(and Han) after being told that they're with Ben. Leia sees Ben die and Luke being devastated. Luke obviously knew Ben pretty well. Luke demonstrates his shooting ability upon their escape. It's not out of the question that Leia vouched for his credibility upon arrival. Maybe even under the assumption that Ben taught him to fly.

Boom simple, the original film already has a good enough reason for why Luke was allowed to join the space battle. No need to over complicate things with the EU, SE, radio drama stuff.

Luke is part of the successful rescue of the Princess and the Death Star plans. At that point he's not just some farm boy.