You both are very responsible and should be commended.
I understand that you both are not ready to have children and for this reason you are working overtime to insure that you won't have an accidental pregnancy. You seem to think that the fact that your boyfriend's siblings had accidents means that they are very fertile and therefore you both assume that your boyfriend is very fertile too.
I wouldn't assume that his siblings are more fertile than other people. You haven't been privy to their sexual habits, and for all we know their accidents were the result of carelessness. So don't make an unwarranted link between them and your boyfriend.
I also want you to know that you are correct that the pill is a sufficient form of birth control all by itself. The small percentage of pregnancies that occur among pill users are not due to the failure of the pill, but the result of reduced pill effectiveness caused by missed doses.
You certainly can use the condom as an additional means of protection when you are ovulating, if that makes you feel better. You should also continue using the condom unless you both have been tested for STDS and the results show that you both are negative.
As for 'pulling out,' as a birth control means, this isn't a recommended practice for two reasons: First, even prior to ejaculation, there is enough semen on the penis to impregnate a woman; and second, pulling out interferes with intimacy. So, I would skip the pulling out all together.
My greatest concern in reading your letter is how anxious you both seem to be about your becoming pregnant. The fear is surely interfering with your enjoyment. So, I think you need to take a hard look at this fear.
It is said that fear often covers an unconscious wish. In other words, you both may unconsciously wish that you will become pregnant. Not being consciously aware of the wish, your unconscious mind only allows you to feel the fear that you will become pregnant.
The fear in this case is fueled by the strength of the wish. Becoming aware of the wish to become pregnant and making peace with the wish may help you let go of some of the fear. You can and should still be cautious, but it would be better for you both to relax and enjoy your sex life more.