You have perfectly explained why Real Madrid wants Trent Alexander Arnold he is incredible attacker with great speed and crosses but his defense is weak as he mostly loses his spot and is not consistent. However you are right that smart coach like Xabi Alonso could fix this by using four defenders to cover for him. It will take time for team to gel but if Xabi can get defense to work well with Trent he could really succeed at Real Madrid despite his current defensive issues.