- If I have already received my Polish birth certificate, does that imply that I have a good chance of succeeding in my application?
It doesn't say much, if anything, about your chances. Even being born in Poland doesn't make a person Polish, so having a Polish birth certificate doesn't mean much more than you have a Polish version of your birth certificate (which, by the way, you do not
need to have in order to be confirmed as a Polish citizen).
- How long does the confirmation stage take?
That is going to depend on how complicated your application is (yours doesn't seem to complex), how many times you are asked to supplement documents, how long it takes you to get those documents, etc etc. It also depends on whether it is still possible to have the 'double confirmation' procedure carried out (i.e. your father is confirmed as a Polish citizen at the same time as you and in the same process) or whether you have to follow the path whereby your father is confirmed as a Polish citizen who has the right to pass on citizenship and then you are confirmed as a Polish citizen.
One more thing while it's in my head, did your grandfather complete or gain exemption from his Polish military obligations?