I have four Humminbird Side Imaging units, I want my data where I am on the boat, not a boat length away.
I have a Side Imaging transducer on my trolling motor and a 360 on my trolling motor.
Side Imaging has to scan to create the picture like images. If you want a full screen sharp image with a trolling motor mounted SI transducer, just keep the motor pointed straight ahead for a full screen update. I have done it thousands of times. When fishing it does not make any difference what the whole screen looks like, the most recent data is at the top of the display, the rest is history.
Set up properly you won't even notice you turn the motor since the scroll speed would be too slow to record the motor turning--just takes practice.
You cannot just turn the motor 90 degrees and leave it that way to get an image, the Imaging pulses have to scan.
While you can save some cost by purchasing a non-SI unit and share SI from another unit, I do not prefer to do it that way.
The Helix 9 and Helix 10 have the exact same size cabinet, just the display size is different.
You asked too many questions in one post !!!!!! There is no simple answer that can cover it all.