In reply to you struggling to turn the boat etc. I think you will find it will come with practice. As I said before, I fount sitting higher much easier. However, I do have proper divers fins rather than the wee things you get with the tube. This may make a big diference.
The line tray puzzles me. I found it sat snuggly against my tummy. Could it be that because you are sitting lower it is being pushed forward??
Scratch that...just looked at your pictures again..you have the latest version with the much thicker seat etc. have a look at this picture of the older style..
compared to yours
Looks like the line tray is longer and narrower and the whole tube a little smaller.