As I understand it, there are high grades of canvassing and lower grades. What we are really talking about is horsehair canvas that is used. I can't imagine a suit that actually has a floating chest piece. This is really what they call half canvas and is used in the chest area to give the jacket some depth and structure. the canvas isn't loose inside the coat. Fully canvassed would be a coat that uses canvassing throughout the jacket and lapels to give more body and resilency. The wool isn't glued to the inside of the jacket but sewn. If done by hand by a very good tailor or suit manf. it will cost substantially more than if sewn by machine.
I disagree with your salesperson. I would rather have a canvassed suit or a half canvassed suit made well rather than a fused suit. There are good fused suits but if you are paying $1500 for a fused suit even if on sale at $750 you are payng too much. I would rather get a canvassed suit for that price. On sale it would be the same. Designer suits are usually fused and expensive. Prada, D&G, Gucci, Valentino Armani Colleczioni all over $1750 each.