Are unknown Pokemon guaranteed to show up in Nearby list?

The Nearby feature will show only the silhouette of unknown/uncaught Pokemon. I noticed that those silhouettes are always at the top of the Nearby list, which is nice if you want to complete your Pokedex.

If there are no silhouettes shown in the Nearby list, does this mean that there are no unknown Pokemon in my vicinity?

(I had a silhouette in my Nearby list but was too busy to mark it or go after it. One minute later, when I was about to go after it, it wasn't in the list anymore. I think that means it wasn't there anymore, so I didn't go to the Pokestop and check.)

If there are no silhouettes shown in the Nearby list, does this mean that there are no unknown Pokemon in my vicinity?

Only at the Pokestops within your vicinity. It is very well possible to encounter uncaught Pokémon that are not in the nearby list.

If you look at the two screenshots below, the account on the left has caught a Drilbur while the account on the right has not. In our location, there are few different pokestops, and as you noted, any uncaught will appear at the top. I know for a fact the account on the right still as plenty of uncaught Pokémon. Since the range will only go so far, it will only display uncaught Pokémon from the visible pokestops.

However, despite not being present at any of the Pokestops, plenty of uncaught Pokémon spawned on the account on the right while not being shown on the nearby list.