But at the same time, they believe that you can effectively baptise your ancestors this way, and that if said ancestors soul accepts the baptism, that's a route to heaven.
So it's not "baptising new members multiple times". It is baptising new members and their ancestors, with the member being a "stand in". They still only have one new member - they obviously have no way of knowing if the ancestors accepted the baptism, and either way they're still dead.
Genealogy basically becomes a route to saving the souls of your ancestors.
From a scriptural sense, the practice is actually based on the concept of modern revelation, where we believe that God has revealed the practice and how it works through modern-day prophets, as was done in the past. The Corinthians scripture ("misinterpretation" is rather open to interpretation) is used simply to point out an instance where it was probably practiced in the churches at the time of Paul.
A living Mormon is baptized in the name of the dead person. This is only done once for a deceased person, which is one of the reasons for the huge record keeping project. The deceased who are baptized are not added the Church roles. Mormons believe that the deceased has the choice to accept or reject the baptism. I am a Mormon, so feel free to ask me any other questions you have.
No, the baptisms for the dead are not included in the count of members. You have to be living and baptised to be counted, and either die or formally request name removal to be removed.
As far as inaccuracy goes, there are lots of people who don't attend and many who don't self-identify as Mormon counted in the membership rolls. I'd guess active mormons are probably somewhere between 1/4 to 1/2 of what's reported.