Not much you can do. There has been a bug in CS6 from the beginning with speed changes in nested sequences and it is obviously so difficult to solve that Adobe has deferred that to sometime in the future. Sometime is not defined.
I think there should be a law stipulating that companies like Adobe must make it clear when a feature in their (pretty expensive) product doesn't work properly, so people like me don't stuck in a situation where they have a deadline they risk not meeting because the software doesn't work as advertised...
I'll have a think about a workaround I guess.. maybe something in After Effects..
£10 suggests you are UK based - try your local trading standards department. Consumer protection legislation applies to software.