Most peds in our area will respect your wishes for delayed vaccine schedules. It is their job, however, to inform you of the risks. If that makes you feel bad, just remember it's their job to inform.
However, it sounds like you have a real reason to delay, so just tell your ped this. (And try not to sound so defensive with your doc. It will probably just put them on the defensive as well.)
Regarding whooping cough, the problem isn't that the vaccine isn't effective, it's that its efficacy reduces faster than expected. Those with the vaccines, even if they catch it, tend to get a milder version. You are correct that it is often adults and older kids, for whom the vaccine is not as effective anymore, that tend to spread whooping cough. This is because the vaccine still protects them from getting the really horrible symptoms, so they don't know that's what they have and they continue to go out in the world spreading the germs around. Herd immunity and getting up-to-date on the vaccine earlier (getting boosters) would help this and provide more protection to infants younger than 6 months old and other kiddos who can't get the shot (for various reasons).