I agree with you on the 'stock' stuff. A 3rd round talent isn't all of a sudden a 1st round talent because of their offseason stuff. They've been that 1st round talent all along. It's just people's perceptions/getting to know more about the player/etc. that changes.
At the end of the day, all that matters is how the teams view these players, not the draftniks and fans.
A prospect's stock is fairly similar to a company's stock, but not quite exact. The player's height/weight, athleticism, etc. + their play on the field, character off the field, etc. are just like companies releasing quarterly/annual reports. What they do "on the field" and "on the books" effects what their "stock" does. The difference is that the 'market' that determines this stock isn't public... the teams determine this on an individual basis pertaining towards their team. Very rarely is there an 'overnight' jump in a players stock... it's gradual even though the casual fan doesn't see this.