Hey, I'm just trying to help. The manual shows the location and function of all of the convergence controls. Your post seemed to express surprise at the function/discovery of one ("there is a pot on the back of the video board if i adjust it, it causes the horizontal convergence to adjust left to right"), so it was completely natural to think that you had not read the manual, ok? Plus, you gave much too serious consideration to the "crt is at fault" hypothesis, which told me that you are relatively inexperienced in these matters. Don't get defensive when someone's trying to help. After all, you were the one asking for assistance.
Sheesh!
Now on to more technical topics. Abandon the mysterious "grid voltage" fault theory. A trinitron has a very simple crt structure. Its convergence circuitry is also very simple. All grids are common to all guns, so you cannot have a failure in a grid that selectively affects convergence of only one color.
There are, however, trinitron crt elements called deflectors. Convergence is achieved in part by applying sweep-synchronous ac voltages to these (electrostatic) deflectors.
When you say that the pot "reads the same" as the pot on the 550, are you referring to a pair of resistance readings (you need to measure a pair for one pot), or to a measurement of the tap voltage? If the latter, does it vary as you rotate the pot? And if so, how does the voltage control range compare with that on the 550?
I agree that a schematic would be nice to have, but you do have the 550 to use for comparison, which is almost as good. Just hang in there, work logically, and you will crack this nut.