First I'll say that it's unlikely that I'll ever get the game, but we'll see what happens. I've played a bit of Dark Souls 1 by the way. Also... incoming wall(s) of text.
1. How do you find out what you have to do, and where you have to go to do it? Do you [i]really[/i] have to just walk in every direction and go as far as you can until you realize it isn't where you're supposed to be? And how do you deal with dying and realize it isn't even the way you were supposed to go? I think I'd feel so crushed if I kept losing my souls if I wasn't even going the right way.
When I first tried Dark Souls 1 in 2014, I barely lasted at all. After the first area with the Asylum Demon, I figured it would be logical to go to the paths closest to me first. IIRC, Blighttown is just off of Firelink Shrine and I quit shortly after trying to go down there.
I've played Dark Souls 1 with a lot of guidance and help now this year, and I feel like I've gone a decent way, but I feel like I've been led down the wrong path and refuse to play without knowing what to do. Uncertainty and risk are some of my least favorite things in life to deal with.
2. How does co-op and PvP work? In Dark Souls 1, I think all you can do is have someone help you with a boss, then they disappear. But I think it's different in DS3. Can you join someone's game as a phantom and walk along with them throughout their journey? And I don't know how PvP really works. I've never invaded anyone or have been invaded in DS1.
I think you'll find that DS3 surprisingly guides you more often. You encounter NPCs more frequently that outright tell you where to go, what to do, and even discuss lore.
The linear map design helps too, which is disappointing since DSI was terrifyingly non-linear if you knew what to do. I zig-zagged throughout the entire world when I got started.
On phantoms, they work as they always have?
NPC Phantoms have always been at the end of an area near the boss, but you can summon other players freely. (As long as they set a sign.)
The conditions have always been the same. A phantom gets banished if:
a) The host dies.
b) The area boss dies. (Which is what I think you mean, since summoning is a lot more restricting if you cleared the area already, but others can still summon you freely.)