I think that possibly the source of confusion is that you are confusing the potential of a charge in an external field and the potential due to the charge itself. When we are considering the work done on the charge the former matters and the latter doesn't. This is equivalent to the fact that when we want to find the force acting on the charge we only care about the electric field produced by external charges, not the field produced by the charge itself.

Whenever we refer to a potential as an absolute value, there is always has to be an implicit "relative to" hidden in the statement somewhere. When we say that the capacitor is charged to 10 volts, that means that the capacitor is charged so the potential difference between the two plates is 10 volts. Now just as there is a vector electric field due to the charges on the capacitor plates, we can also describe a scalar potential field due to the charge. The vector electric field is the gradient of the scalar potential field. Consequently, just as the electric field is a property of the charge on the plates alone, so is potential field.

The potential field is defined only to within an arbitrary additive constant. Conventionally, this constant is usually assigned to make the potential at infinity equal to zero, if at all possible, but it doesn't really matter. The positive plate will have a potential 10 volts greater than the potential at the negative plate. The potential at the positive plate is the maximum of the potential field; the potential at the negative plate, the minimum. Halfway between the plates, the potential is 5 volts below the positive plate and 5 volts above the negative plate. As we go farther and farther away from the capacitor, the potential approaches this in between value. This is field gives the potential of an external charge due to charge on the capacitor and *it is completely independent of the magnitude of that ***external ** charge.

Now, when you say that your particle is charged to 100 volts, you imply that if the charge were less so would be the potential. The potential of a charged particle due to its own charge is not something that is that commonly considered. However, sometimes we might consider the potential difference between the surface of the particle and a point infinitely distant from the particle. That is the best guess I can make for what we might consider to be an intrinsic potential of a charged particle. However, I need to emphasize that if that's the case, we are talking about the potential due to the particle's own field, and this quantity is irrelevant to how the particle responds to an external electric field.

To look at what happens to a charged particle moving through an external electric field we are only interested in the character of the field and the value of the charge. The electric force on the charge at any moment is equal to the electric field vector at the particle's location multiplied by the particle's charge. The work done by the electric field if the particle moves from point A to point B is equal to the electrostatic potential difference between point A and point B multiplied by the particle's charge. That's it.

Now, if we consider the case of a charged particle moving between the plates of a capacitor, and we give that moving charge a magnitude *comparable to or greater than * magnitude of the charges on the plates, we will have a scenario that will not be adequately approximated by the electrostatic description of a charged capacitor. In this case, as the particle approaches it will exert a sufficient force on the charges on the plates to change the charge distribution significantly from what it was when the particle was distant. This means that the electric field produced by the capacitor will be changing moment by moment and the whole thing becomes an electrodynamics problem. People take graduate courses to learn to solve this sort of problem properly. So under that circumstance, the charge on the incoming particle matters more than what I suggested above. However, even in that case we would expect the particle to be attracted by the opposite sign plate and to be repelled by the same sign plate. The particle will still be deflected in its flight.

If on the other hand we are considering a charged particle with a charge magnitude that is small compared to the charge on the capacitor, the electrostatic capacitor is an excellent approximation. The electric field that the particle is exposed to will at all times be well approximated by the field of the capacitor with no external charges present.

All this isn't to say that there are no instances in which the *combined * field of the particle and capacitor (and similarly the combined potential due to those fields) is a quantity of interest. But those circumstances would be when we are interested in the effect on a *third * hypothetical entity. If we are interested in the effect of the capacitor on the particle, we need to keep the particle's field well out of the discussion.