What word best describes talking sweetly but in a sarcastic manner?

The closest single word may be "gibe". It is the translation of the Spanish, "puya" which literally means "tack" or "thorn," but which is also used to describe a carefully crafted comment that appears to be a compliment, but is actually meant to be deflating to the one for whom it is intended (the effect of a carefully-placed tack).

For example" "That new top is perfect for you. It's remarkable the way it seems to cover those extra pounds."

"Congratulations on the new job! will you be moving out of you parents house soon?"

I'd go with saccharine

too sweet or sentimental : sweet or sentimental in a way that does not seem sincere or genuine

This derives from early use of saccharine, which was one of the first artificial sweeteners. While the resulting taste was sweet, it also had an overtone which made it fairly easy to realize that it wasn't really sugar.

I'd say honeyed. A honeyed voice is sweet and pleasant to listen to but is not sincere. I'm not sure if this is accurate enough - probably only describes the tone - but hopefully it helps. Combine this with "condescending" and I think it'll pass. (Sorry, first time answering a question on here)