When sitting with your legs open do a karate chop with your hand on the centre of the apron as you sit down. Thus forcing the material between your legs. I wouldn't cut the under apron as it is there for a reason, when the wind blows you have full coverage.
In the wintertime a kilt is rather warm because of the airflow. Which is warmer, gloves or mitts? If you answer mitts, you're right! same with a kilt compared to trousers. It does help wearing long kilt hose but I have hairy legs so it doesn't much matter to me.
Cheers
Robert