Is it a sin not to inform one's parents about booking them an 'Umrah trip as a surprise? And what is the ruling on wearing the niqab in Makkah despite the parents' refusal due to security concerns?

There is no harm in you performing the 'Umrah procedures for your parents without their knowledge. Covering the face is obligatory according to the most preponderant opinion among scholars, and obedience to parents is not required if they forbid you from wearing it. If exposure to harm is a reality, then there is no harm in taking the concession to uncover the face to ward off hardship. However, once the excuse is gone, you must cover it.