Good questions that you ask. I hope I can contribute to the good answers.
For the 1st part, as Ches said, do not fasten to the frames. If you do it creates "hard" points that will telegraph through the fair curve as angle lines lying perpendicular to - across - the direction of the curve. What you want is a fair, smooth curve. Almost always this is one with a constantly changing radius. It is that, after all, which creates the fair lines that we perceive as "sweet". Touch-points caused by contact with the frames interrupt the fair curve and make it into a series of shorter curves separated by angles. It will sorely dissatisfy you.
For the 2nd part, installing more "sheers" as you call them (actually these are "battens" when located between longitudinals of sheer, chine, keel, etc.) will disturb the flatness of the plywood plank that is across the direction of its curve. It will probably try to set a convex or concave shape across the direction of curve. It can be done to an extent. When you accomplish that, you have crossed into the unholy region of "stressed plywood" construction. Some aircraft designs have used this construction, called "monocoque" because of its great strength to weight ratio. You can eliminate most of the frames and battens with this structure, it's so strong.
I'll try to explain that better.
A flat sheet of plywood bends willingly (unstressed) in one plane, only. Plywood in this mode is actually a section of a cone in any cross-section. Think about it. A cone made of a flat sheet of paper is curved in only one direction, right? At any point on the curve, a line that is perpendicular to the curve, that is, from the top of the cone to the bottom, is totally flat - a plane. The same with plywood in your boat. You can lay a long straight-edge across the curve of the panel and, unless it is forced into a slight compound curve (two directions or more) the straight-edge will contact across the entire panel.
Therefore, unless the additional battens you install are in precisely the correct plane, perpendicular to the curve, longitudinal angles, concavity or convexity will result. It will appear not fair - in the boat building sense of that term. The best plan is to stick to the designer's plan. Don't worry about strength. In fact, additional members, hence weight, do not always yield greater strength. Wood is already incredibly strong. Made into an engineered product like plywood enhances even that.
I hope this helps. It is somewhat abstract and hard for me to explain it well.
Paul Miller in Memphis, TN
"Yeah, I had lunch with him last week at the Cracker Barrel out on I-40."