There is no way to pass botulism spores to the fetus by applying honey to your scalp You do not need to worry. This will also not help at all with curing seborrhoiec dermatitis of the scalp Use an alternating combination of shampoos made for this such as Selsun Blue and Neutragena T gel. You need to work them into the scalp vigourously and leave on for ten minutes before rinsing. Rarely do you need to worry about topical application while pregnant. The skin, even while not intact is a good barrier.
Even if it did, adults are generally immune to botulism. You would need to be infected to infect the fetus. It is still important not to use unproven treatments that are potentially harmful. Ask your Dr first.
Yes that is the case You do need to be careful in pregnancy as many supplements billed as natural are very unsafe.
Most honey is unsafe to eat while pregnant but avoid natural and unprocessed honey.