Yeast infections are extremely common during pregnancy, and the best medication to use is a 7-day treatment like Monistat 7. The 7-day treatment is better than using a 1-day or a 3-day treatment because yeast infections are harder to clear-up with pregnant women than non-pregnant women.
If some doctors are saying not to use anything and some (like mine and yours) are saying it's fine, go with what makes you feel comfortable and gives you peace-of-mind.
Like you, I was advised to do was to not insert the applicator far (only insert so that the medication barely enters) and then use the topical Monistat on the external area. Apparently the risk is that the applicator could damage the mucus plug & medication could be absorbed, so it's important not to place the applicator too far or place the medication too far.
I experienced a burning sensation when using the product when pregnant, so I used it externally for a day or two and then gave up. Thankfully the infection went away on its own.
The best advise is to check with your doctor again (about your worries) and use it externally to be ultra-safe. Since you're really only experiencing symptoms with sex, you may want to refrain from the medication if you are worried about side-effects.