Shouldn't the requirement be 8 ranks in Concentration, not 9? You mention entering after 5 class levels.

If you can stomach the delayed entry due to lost BAB (perhaps by taking other PrCs immediately at level 6 and then taking Valkyrie later on), a single level of Stargazer for Complex Formula meets the evoking requirements, rather than needing two levels of any other class. Is this intentional?

With the standard ToB disciplines, entry is impossible until after level 6 as there are no 2nd level stances to take, thus requiring a 3rd level stance instead. I'd suggest changing it to a 2nd level maneuver instead, thus also not impeding a Warblade entry more so than other ToB classes and being more consistent with other ToB PrCs.

Other than the issues with the entry, the Valkyrie class looks pretty darn good.