1. Fighting from the doorway.
Why do the monsters fight? Intelligent ones should run and get reinforcements until the entire complex is aware of the PCs presence, where they can attack with overwhelming numbers. My players regularly attack fleeing enemies to keep them from warning others.
2. Scouting.
This can actually be done quite well, keeping the whole group engaged, if handled properly by both you and the group. If they send in a scout, describe everything to them and the group at the same time. When a decision point must be made, such as a room, encounter, or an intersection, the scout should return to the party, who moves together up to the decision point to make the decision together without you re-describing everything. While technically only 1 PC is "active," unless they get ambushed (which is REALLY bad for the scout), it's not really any different than you describing everything for the group. They explore the important parts together, but the scout gets to make use of their stealthy abilities.
3. Caution.
This is really group dependent, and I'll admit old school gamers like me are a bit on the paranoid side. Earlier editions were fraught with insta-death traps, and that can be a hard habit to break. Some players that are used to newer editions often charge straight in, throwing caution to the wind, because they expect the encounters to be balanced. Best solution is to talk to the group in session 0 to discuss which style of game you want to play; heroic fantasy or gritty realism. In a heroic fantasy game, players are more likely to build brash characters, or foolish ones that make poor choices due to a flaw.