If all the power or only a small farction is actually used makes a big difference. The problem with the assumption all the theoretically possible power would be added to the kinetic energy is that it is rather hard (essentially impossible) to do this.
With a 100 kg vehicle to add the first 0.5 Ws to the kinetic energy it would need a speed of 0.1 m/s. If the sail would provide a full 500 W to add to the kineatic energy, this would be only 1 ms to reach 0.1 m/s. It is only a low speed, but 0.1 m/s / 1 ms is still 100 m/s² and thus a bit more than 10 times gravity for the acceleration needed (on averge). Things get even more carzy when looking at lower speed / shorter time. So there must be an error in the calculation.
The error in this example is in the idea that a "sail" would be 100 % energy efficient in converting to kinetic energy. Energy is still conserved, but most is converted to heat and not to the kinetic energy of the vehicle.
I somehow have the feeling the concept of force is not really understood. Using a power for the drag is an indication. Drag is a force and not a power.
Except from this error I have not seen a problem with the units. The desire to do the calculations with examples instead of the letters is often found with beginners who try to use intuation instead of math. However even beginners are usually way better in realizing that they may be wrong.
We are discussing an ideal case so no friction losses.
And yes it will take less than one ms to accelerate to 0.1m/s 0.85ms as there are 600W when you start with 10m/s wind speed and 1m^2 sail ideal case.
But 600W is not infinite or incredible and less than 1ms means there is not much energy at all
You are maybe concerned in real life about the sort of forces that will require but you probably forget that in real life all materials deform hopefully just elastic deformation if vehicle was designed correctly.
Think about just a wheel with an air filled tire and perfect brakes applied so that wheel can not rotate. Due to tire elasticity you can still push the vehicle without needing infinite force
In ideal case this elasticity and thus any energy storage in them is ignored but in real life you can not get rid of them the same way you can not get rid of friction.
So that 0.5Ws (half a joule) is just nothing and will likely be absorbed by some part deformation like mentioned rubber on a wheel.
So from theory you think there are huge forces but in practice those are fairly small since they are dampened by all the elastic deformations in a system. Even steel will have some amount of deformation and yes that require higher forces but still nothing close to infinite or what you will get when you try to do a theoretical ideal case.
That is why power and energy are much more intuitive to use as they will not seem ridiculous like around 600W for less than 1ms or 0.5Ws seems super small.
A sail is the most efficient device you can use if you want wind power converted into kinetic energy and an ideal sail will be 100% efficient that is why this calculation demonstrates that a 100% efficient wind powered vehicle can not exceed wind speed directly downwind and thus any vehicle no matter how it is build can only exceed wind speed directly downwind if it has some sort of energy storage device or a external energy source.
Even a real sail will be fairly close to 100% efficient same way a wheel is super efficient and very close to 100%. A propeller by contrast is super inefficient but the advantage in this particular case is that propeller can increase the pressure differential and thus a way of storing energy.
I feel you are the closest here in understanding this. Hopefully my clarifications on why those large theoretical forces are of no concern in a real system are good enough if not let me know and I will try to add more details.
I'm fairly bad at explaining things.