Hi Garry, and a warm welcome to the forum.

It's great to have another Scott Bonnar Model 45 owner on board, as we all like to learn new things and tinker here.

You have posted in the correct area, well done!
Now to your questions;
A Scott Bonnar manual and parts list is available in the Lawnmower Parts List and Repair Manuals topic in the Questions on Lawnmower Frames area. Access to this area is by subscription. Just click on the second post in any topic on the main forum page, for instructions on how to access...easy peasy!
(a) Cylinder reel moving whilst the cutter clutch is disengaged is definitely not OK....Providing that the clutch is serviceable, it just needs adjustment.
(b)It is perfectly OK to ride the clutch, in fact, they are designed to do exactly that....Just squeeze the handle enough to give you a steady walking pace that feels comfortable to you.
(c) Without seeing the machine, I would suspect it is the later model of 45 with the twin rails for motor mounting. These are renowned for cracking due to vibration. There are various methods of repair, we will go through those at a later stage. Could you pleas post some pics of the Scotty, as we all like to see them, and will help us diagnose your prob.
Once again Garry,