Generally it's a great idea that you continue in trying to make more extensions, but does it make sense to try to rebuild something, what is already available? Why not trying to create something new?
As far as I can see, there is still a looooong way to go to get this running... good luck!
Btw. what I'm asking myself is, after taking a look at the logcat output, why does your methods
SubscriptionDetails use TaifunBilling as tag instead of using your own tag? Strange, isn't it?