How is a 0-100 run done?
The fewest number of gears possible will be used, because that's the fastest way to get to a given speed. Engines typically put out their highest power in a certain band of RPMs, so you will accelerate as much as you can in first gear (to the red-line, or to a certain RPM where you know the power starts to decrease), then shift to second and repeat. In these runs, you might only get to third gear - that all depends on the transmission and engine combo. Unless you want a very slow time, you're not going to drive like normal where you would shift much sooner. Normal driving is for comfort, less wear on the engine, and better economy - not for performance.
Is a linear curve valid?
No. The power curve, aerodynamic resistance, and any number of other factors are not linear so acceleration won't be either. You can make an informed guess that's linear or a fixed curve, and it might be close, but it won't ever be correct for all cars.