Yes, I do think your ranges are number # 1 the highest ever reported here for B6.

I'd stop the complex pronto.

As to what is going on? Well, some people cannot convert B6 to active P5P, so it may build up. But still, I've seen ranges in the low 100's on the net here and elsewhere. 700+ is mega elevated.

You can take Benfotiamine daily (the better B1) and methylcobalamin like you are doing. 600 is in the okay range. But a bit higher may be what your body could use more efficiently. This is not harmful, and worth a try. (That range you gave going down to 180?...that is horrible... people can have significant problems
at 180.)

Yes, high B6 has been found in autistic children, and the reasons are not clear.

I'd make sure you are getting enough magnesium, as magnesium works with B6 in many enzyme systems.
Magnesium is lost with prednisone therapy.
This is my magnesium thread... foods, the right choices for oral and some topical creams if you prefer:

http://neurotalk.psychcentral.com/thread1138.html

And yes, lab errors do occur... I'd get retested. Sometimes factitious highs come from the red cells bursting if the sample is not taken properly, or handled properly. This is common with potassium readings, since the red cells hold lots of potassium too and can burst and release it into the serum.
There is a place called Spectracell that tests only red cells and tends to be more accurate. But I don't think insurance pays for them.

http://www.spectracell.com/

The chemo agents damage your DNA to some extent. So consider taking some acetyl carnitine to help with that damage. It will enhance your mitochondrial actions and improve cellular functioning if damaged by the chemo. 1-2 grams a day, divided doses may help. Start at 500mg a day and work up if you need to.