No, there are some factors controlling visitor appearance at play, but they don't seem to be understood. I've had half a dozen pocket world fortresses in 0.42.X, all with nice taverns, temples, and libraries, and have received a total of 1 (human) visitor. Since all but the one with the visitor were intended to try out the new stuff in 0.42.X that was a bit of a let down, so I generated a world two steps larger, and received large number of visitors (including nobles from my own civ, at the edge of the world).
I did ask about the conditions controlling visitors at the time, and didn't get any answers...
My current guess is that visitors are drawn from populations of other civs, much like siegers are. In order for civs to produce visitors they probably have to be sufficiently large to "afford" to have lazy bums traveling the world.
When I started the single visitor fortress the nearby human civ had 136 humans and 187 others (mostly goblins), but when I checked several decades later they were down to 99 humans and 4 goblins (I think), while they're currently up to 105 humans and 4 goblins. The largest elven civ had about 109 elves at embark, but the largest one currently has 209 (I assume though assimilation of a one of the other sites). All the goblin civs have imploded, so while the world has 27000 goblins, the two non failed goblin civs have 0 goblins (all the others are living at sites not controlled by any civ).
A wild guess based on the above is that a civ would need to have 300+ members to send out visitors.
I didn't mention dorfs above, because my civ is dead. Every dorf in existence lives in my fortress.
Visitors don't care about sieges, so they can arrive in droves during sieges. Military visitors fight/are fought by goblins, while civilians are completely ignored (undead attack everyone, of course). The "path" required is just a path to the tavern/library/(temple?), and that isn't actually required either: if visitors arrive and there isn't a path they'll just stay by the map edge, and start to move when you lower the drawbridge (I believe migrants behave the same in that respect).