I will say, in my calculations I forgot the final 2 back to back suicides. So now back to math.

Your estimated speed outcome is off by a few numbers. Mine being an exact 581.025. I do my math using a straight .25 equation like in your CS. But at this rate thats a miniscule measurement.

Unfortunately Xaviers acceleration is based off his strength. Which is still higher than yours by 500 points. I used the average black belt strength for this measurement. As you have stated Strength is just a secondary stat.

Now when we compare your speed measurement to your strength measurement you see that your speed relies 24% on your strength. I adjusted your speed in MY calculations to 590 to better suit both sets of equations. Now what we will say, is that 24% is a trained number. Meaning Auz worked to achieve that in even average sword fights. 

Your base stat of 412 Strength when 24.5% is taken out is roughly 100.

Now we take Xaviers strength stat of 3000. Xavier focused on a different type of speed than Auz, acceleration vs top speed. So we will say his speed relies 18% on his strength. Bringing his Speed stat to a firm 540. With an increase at the beginning of any focused movement.

So I would say in this one instant, speeds are matched. Yes Auz can move faster, but Xavier can get moving faster. Auz could beat Xavier in a sprint, but Xavier would get off the line first. 

End Results: Auz - 590 Spd 2455 Str
Xavier - 540 Spd 3000 Str

The strength difference is where Xaviers superior acceleration comes into play. He has more muscle to move himself harder, faster. But most of the strength is in his arms so it wears thin in long distance.

As for the attack, Xaviers body is filled with the blood that creates his weapons. It moves on its own accord to defend itself when being wounded. There would also be blood covering his neck already because he never closed the throat wound. Also Xavier can call on a small amount of the metal on his chest to defend as well, softening the blow before it can connect with the neck. I wholey expect a partially severed head, but the vertebrae would stand strong. Xavier is a tank first and foremost.  I only say a skull blow would do it because Xavier hardly covers his face/head with the metal. So a direct blow would be deadly. But here there is plenty of defense on hand for the neck blow. Earlier in the fight would be a different story.