With a button, rather than a preprogrammed delay... that's a smart way to do it. You don't have to worry about how long, exactly, the companions take to teleport to the player, since the player will figure that out on their own.You are pretty much correct. I plan that to find new dungeons, a player must either:
1: Buy the warp page from an NPC, the cost I have yet to determine
2: Unlock a dungeon by quest. Most of these ones will be "Legendary Dungeons", as in they'll be cooler and be a devolopment of plot
These warp pages teleport you into the buffer room for that specific dungeon only, and you just press the button to enter the dungeon.
I'm actually really looking forward to seeing this all come together, even though I'd generally rather build contraptions than fight monsters.