Vatican II did not change Church doctrine about rebaptism. Before Vatican II and after, the Church doctrine was St. Paul's doctrine "One Lord, one faith, one Baptism". The Church forbids the repetition of the sacrament, once validly administered, even for converts.
Official Baptist doctrine comes from Scripture. Baptists being independent, there is some lee-way on non essentials so you may see some Arminian Baptists for example and some some Calvinsts, some pre Trib Rapture folks and some Amillenial. Scripture is where most of the doctrines of the early Creeds came from (yes, after some debate over what Scripture means).
But you neglect to mention that some doctrines dealing with the essentials of salvation varies among Baptists. For example, the Calvinist doctrine of predestination. By being independent, Baptists who read the same scripture arrive at opposing doctrines.
Catholic doctrine comes from Scripture plus Catholic tradition. The latter moves about fairly frequently depending upon what pressures are being placed upon the church at the time (such as will we see a softening on the stance on contraception? which some would consider a mortal sin?) or who happens to be in the Papal/Cardinal seats at the time.
Could you please provide some examples of Catholic doctrine that has changed because of pressures being placed upon the church or because of who happens to be in the "Papal/Cardinal" seats at the time. As far as softening on the stance on contraception, that is purely speculation on your part. And as far your comment that some consider contraception a mortal sin (or not) does not change Church doctrine or teaching.
Calvinism verses Arminianism is NOT an essential. It is a secondary. Salvation by Grace through faith alone is a primary.
When I have more time I will answer you further. But, your point regarding my point on mortal sin backs up my previous point that just because somebelieve it, teaches it, etc., it is not official until it is official. Now if it were in the catechism, I'd say that is more official. If it just something in Benedicts writings as a Cardinal, it isn't official doctrine.