I'm only a green belt in HDGD but we started cartwheel practice at white belt. As I could already to a cartwheel it was easy for me. Especialy since my glasses keep sliding off during the cartwheel and I now hold my glasses in my sword hand while doing the cartwheels. Making a fist on which you "lean". Most of the time I actually do (almost) a one handed cartwheel with 95% on my non sword hand and 5% on my sword hand. As for doing the cartwheel with a kagum or jingum, the idea is that the blade never hits the floor. When you hold the sword in your hand and cartwheel on your fist the blade should be of the floor. At least when you do it correctly it should. As for everything, practice makes perfect, some (private) gymnastics lessons on the subject could help.