Dammit, I completely missed that the point of those was to shield Ormund from the plebeian stench. Made me check the scene on youtube. The guy still has more restrain than I thought he did.Well they did make a point of showing those gigantic incense "dispensers" at the camp they were sitting at lmao
Obviously, we can say they are hatched with magic, but we're not exactly certain if blood magic is always required or not, given we know that's how Dany hatched hers. As far as how Dreamfyre (or possibly Vhagar's) eggs ended up in Pentos is pretty much plotted out in Fire & Blood. The short of it is that Dreamfyre's original rider, Rhaena Targaryen (Aegon the Conqueror's first grandchild) had a female "friend" from House Farman named Elissa Farman, that got up to a whole bunch of hijinks and adventures. She basically stole a clutch of eggs from Dragonstone and it's generally assumed they were Dreamfyre's due to her relationship with Rhaena as well as the she-dragon's reputation for being prolific.
Alys Rivers finding dragon eggs at Harrenhal isn't really talked about directly in Fire & Blood, but there were rumors about her having a dragon. IIRC it was treated more as hearsay since her having one wasn't really of much consequence in the story. But Rhaena Targaryen spent her later years ruling Harrenhal so the idea that Dreamfyre could have laid eggs there isn't farfetched.
What throws me off is that in theory dragons at some point existed in nature (though technically it's not impossible some valyrian magic or breeding created the). In my headcanon it makes more sense that magic weakened in the world as a result of dragons disappearing if they were already just part of the world. This makes less sense if they were breed or magic-ed into existence by valyrians with magic. Of course with magic and with GRRM making overt efforts to keep things as nebulous as possible anything could go. Blood magic is something valyrians were familiar with though I am not sure of what Danny did which counted as such. Technically mas... mirri... durr or whatever her name was used blood magic prior but this was merely in proximity to the eggs, not even used on them. And then Danny went into the flames with drogo's corpse and the witch and the dragons hatched. It also looks like targaryens could hatch dragons by the time time of the dance and it doesn't look like they were skilled with blood or any magic. At this point I think dragons stopped hatching simply because their numbers were reduced enough that there wasn't enough magic for them to hatch until danny accidentally stumbled into a mass of it or something.
The bit with Farman sounds like a pretty strong theory. It definitely looks like Alys and her dragon will be somewhat prominent in the tv show at least. Lots of focus on her and aemond.