I also find the japan to be very tough. Someone once told me "relax on the grab," and it is great advice. Focus on getting the pop and set correct and follow the earlier advice about trying to get the japan on the side. Relax and don't force the swinging of the legs. I have the opposite problem with my japans now, Sometimes they kinda flatten my spin and freak me out like I am rollin into a linken
I spin right and I grab Japan with my right hand. That way the right arm and shoulder is going the same direction as the rotation. I also find it's the easiest grab to hold with a 3, and helps my body get more compact in the air, but when I started doing them I had the same problem as the other guy that would start doing a lincoln, going sideways in the air, but if you try to keep the left arm down or even try a double Japan I find that keeps you on axis.
Japan was the third grab I learned. I just did safety first, then thought of blunt as just safety but farther back, and then just thought of japan as blunt with the other hand but the same ski, and for some reason in my mind it just clicked that way and was easy.
I find it hard to get both my legs up for mute grab without leaning forward though, they always come up sketchy or with one leg still extended.