There's a few things controlling when you can summon someone (even with the password on).
First off, when you have password summoning on, the soul-range system is overridden, so your level 100 friend can come help your level 20 character, they will just get a normalizing damage reduction.
Second, if you are in an area where you have defeated all bosses in that area, you cannot summon anyone, regardless. You used to be able to forgoe this in Dark Souls and Dark Souls II with the Small White Sign Soapstone, but that doesn't seem to be the case in 3, as I haven't seen that item in-game. The exception to this rule is that you can do PVP summons (red sign soapstone, etc.).
Third, the summoner must have used an ember (or defeated a boss) restoring them to be a Host of Ember.
Finally, Dark Souls also enforces a "summoning cooldown" at times, limiting when you can summon your friend.
If all these conditions are met, you should be able to summon. Keep in mind it can take up to a few minutes (in my experience) for summoning signs to appear.
Also, just a sidenote: the Dark Souls netcode is sometimes a bit buggy. There are certain areas I cannot summon my friends in, but they can summon me, regardless of us having the same progress. While it's not foolproof, sometimes both the summoner and the summonee restarting Dark Souls 3 completely.
Yoel of Londor will die after you Draw out True Strength five times (aka free levels), or reach the catacombs, whatever comes first. Yuria of Londor will show up after he dies, and will sell the same items he did, but doesn't have the Draw out True Strength option. If you want those five free levels you need to make sure to do so before hitting the catacombs.
The only way to bring him back is going to NG+.
Best Answer
PvP-related areas have a restricted amount of summon slots because of that area's covenant invaders take up said slots. You can lift that restriction using the Dried fingers as you noticed. Normally you can only have 4 players in a world (you + 3 others). Using the fingers adds 2 slots.
The Watchdogs of Farron take up slots in Road of Sacrifice and in Farron Keep. The Aldrich Faithful take up slots in Anor Londo.