I can try to explain “Nous voici partis dans les rêveries“, and that just means “We are now daydreaming“. Maybe all those numbers and the way Joseph de Maistre associates them with various concrete things makes the author feel dreamy, prompts the author to start daydreaming.

As for the mint-smelling milk (not menthol, but mint), no idea.

“rêveries” comes from “rêve” which means “dream” in English. So the use of the verb “partir” here is more metaphorical, they’re not physically moving, it’s their thought who are going into dreams.

About the milk, the correct translation of “menthe” would be more “mint” than “menthol” (but still, I’m not sure why milk would smell mint 😉 ).

(1) If you’re wondering about the syntax, just think of “voilà” as taking a direct object much the way verbs do in the imperative: “Nous voilà” = “There we are”, “Les voilà” = “There they are”, etc.

A typical rendering of “Nous voilà partis dans les rêveries” might be “There we are, disappearing into (our) daydreams.”

But this is a good example of the versatility of “voilà” (and voici), since an equally fair way of putting it might just have another phrase to move time along in the passage: “And then we’re gone, disappearing into (our) daydreams.”

(2) Your question about “aller boire du lait” isn’t clear enough for me to be sure where the issue lies. Can you clarify?

(3) The verb “sentir” takes a direct object (for reasons as arbitrary as any involved in discussions about why verbs don’t have the same object structure in English and French). Feel free to insert “like” or “of” in your English translation: milk “that smells like mint” or “that smells of mint”.